feat: honour LUNITS / LUPREC unit formatting + INSUNITS scaling on insert

Property-panel coordinate display and block-insert scaling were both
hard-coded, ignoring the document's stored unit configuration.

- LUNITS / LUPREC: edit_prop() now formats values through a thread-local
  UnitContext seeded by refresh_properties(). Decimal / Scientific /
  Engineering / Architectural / Fractional all render correctly without
  threading the document handle through every entity properties builder.
- AUNITS / AUPREC: format_angle() helper available (decimal degrees, DMS,
  grad, rad). Callers that already format angular values via radians can
  switch over without touching the helper signatures.
- INSUNITS + MEASUREMENT: xref's source INSUNITS is carried onto the
  host BlockRecord.units during merge. commit_entity() then scales new
  INSERTs so 1 source-unit maps to the host's INSUNITS length. When
  either side is unitless (0) MEASUREMENT acts as the fallback
  (0=Imperial / inches, 1=Metric / mm), matching AutoCAD's rule.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/properties.rs

@@ -15,6 +15,19 @@ impl H7CAD {
         let edit_buf = std::mem::take(&mut self.tabs[i].properties.edit_buf);
         let selected_group = self.tabs[i].properties.selected_group.clone();
 
+        // Seed the per-thread unit context from the document header so the
+        // entity property builders (which only see f64 values) can format
+        // lengths/angles per LUNITS / LUPREC / AUNITS / AUPREC.
+        {
+            let h = &self.tabs[i].scene.document.header;
+            crate::entities::common::set_unit_context(crate::entities::common::UnitContext {
+                lunits: h.linear_unit_format,
+                luprec: h.linear_unit_precision,
+                aunits: h.angular_unit_format,
+                auprec: h.angular_unit_precision,
+            });
+        }
+
         let layer_names: Vec<String> = self.tabs[i]
             .scene
             .document
@@ -428,6 +441,34 @@ impl H7CAD {
             entity.as_entity_mut().set_layer(layer.clone());
         }
 
+        // INSUNITS: when inserting a block whose BlockRecord.units differ
+        // from the host's header.insertion_units, scale the new INSERT so
+        // 1 source-unit equals the matching host length. When either side
+        // is unitless (0) AutoCAD falls back to MEASUREMENT (0 = Imperial /
+        // inches, 1 = Metric / mm); honour the same fallback.
+        if let acadrust::EntityType::Insert(ref mut ins) = entity {
+            let header = &self.tabs[i].scene.document.header;
+            let measurement_fallback = if header.measurement == 1 { 4 } else { 1 };
+            let host_raw = header.insertion_units;
+            let host_units = if host_raw == 0 { measurement_fallback } else { host_raw };
+            let src_raw = self.tabs[i]
+                .scene
+                .document
+                .block_records
+                .get(&ins.block_name)
+                .map(|br| br.units)
+                .unwrap_or(0);
+            let src_units = if src_raw == 0 { measurement_fallback } else { src_raw };
+            if src_units != host_units {
+                let ratio = insunits_to_mm(src_units) / insunits_to_mm(host_units);
+                if ratio.is_finite() && (ratio - 1.0).abs() > 1e-9 {
+                    ins.set_x_scale(ratio);
+                    ins.set_y_scale(ratio);
+                    ins.set_z_scale(ratio);
+                }
+            }
+        }
+
         crate::scene::dispatch::apply_color(&mut entity, self.ribbon.active_color);
         crate::scene::dispatch::apply_common_prop(
             &mut entity,
@@ -637,3 +678,32 @@ fn merge_prop_value(
         _ => left.clone(),
     }
 }
+
+/// Convert INSUNITS (DXF group 70) to millimetres.
+/// 0 = unitless / unknown: returns 1.0 so the caller treats it as "do not scale".
+fn insunits_to_mm(code: i16) -> f64 {
+    match code {
+        1 => 25.4,            // Inches
+        2 => 304.8,           // Feet
+        3 => 1_609_344.0,     // Miles
+        4 => 1.0,             // Millimeters
+        5 => 10.0,            // Centimeters
+        6 => 1_000.0,         // Meters
+        7 => 1_000_000.0,     // Kilometers
+        8 => 0.000_025_4,     // Microinches
+        9 => 0.025_4,         // Mils
+        10 => 914.4,          // Yards
+        11 => 1.0e-7,         // Angstroms
+        12 => 1.0e-6,         // Nanometers
+        13 => 0.001,          // Microns
+        14 => 100.0,          // Decimeters
+        15 => 10_000.0,       // Decameters
+        16 => 100_000.0,      // Hectometers
+        17 => 1.0e12,         // Gigameters
+        18 => 1.496e14,       // Astronomical Units
+        19 => 9.461e18,       // Light Years
+        20 => 3.086e19,       // Parsecs
+        21 => 304.800_609_6,  // US Survey Feet
+        _ => 1.0,
+    }
+}

src/entities/common.rs

@@ -1,7 +1,126 @@
+use std::cell::Cell;
+
 use glam::Vec3;
 
 use crate::scene::object::{GripDef, GripShape, PropValue, Property};
 
+/// Linear / angular unit format pulled from the document header so the
+/// per-thread properties pipeline can format values consistently without
+/// passing the document through every callsite.
+#[derive(Clone, Copy, Default)]
+pub struct UnitContext {
+    /// LUNITS — 1=Sci, 2=Decimal, 3=Engineering, 4=Architectural, 5=Fractional
+    pub lunits: i16,
+    /// LUPREC — decimal places (linear)
+    pub luprec: i16,
+    /// AUNITS — 0=Decimal degrees, 1=DMS, 2=Grad, 3=Rad. Surfaced via
+    /// `format_angle`, which is read on demand by code that already
+    /// formats angular values via radians.
+    #[allow(dead_code)]
+    pub aunits: i16,
+    /// AUPREC — decimal places (angular)
+    #[allow(dead_code)]
+    pub auprec: i16,
+}
+
+thread_local! {
+    static UNIT_CTX: Cell<UnitContext> = const { Cell::new(UnitContext {
+        lunits: 2,
+        luprec: 4,
+        aunits: 0,
+        auprec: 0,
+    }) };
+}
+
+/// Set the per-thread unit context. Properties helpers consult it when
+/// they format f64 values into display strings.
+pub fn set_unit_context(ctx: UnitContext) {
+    UNIT_CTX.with(|c| c.set(ctx));
+}
+
+pub fn unit_context() -> UnitContext {
+    UNIT_CTX.with(|c| c.get())
+}
+
+/// Format a linear length using LUNITS / LUPREC. Architectural / fractional
+/// produce "n'-d/D"" style strings (1 unit = 1 inch); decimal / scientific /
+/// engineering / Windows-desktop fall back to plain decimal at LUPREC places.
+pub fn format_length(value: f64) -> String {
+    let ctx = unit_context();
+    let prec = ctx.luprec.max(0) as usize;
+    match ctx.lunits {
+        1 => format!("{:.*e}", prec, value),
+        3 => {
+            // Engineering: ft-inches, decimal inches.
+            let sign = if value < 0.0 { "-" } else { "" };
+            let abs = value.abs();
+            let feet = (abs / 12.0).trunc();
+            let rem = abs - feet * 12.0;
+            format!("{}{:.0}'-{:.*}\"", sign, feet, prec, rem)
+        }
+        4 | 5 => {
+            // Architectural / Fractional — n + fraction with 1/2^p denom (1
+            // unit = 1 inch). Use 6 as a moderate denominator power so the
+            // result reads like 1/64".
+            let sign = if value < 0.0 { "-" } else { "" };
+            let abs = value.abs();
+            let (feet, in_rem) = if ctx.lunits == 4 {
+                let f = (abs / 12.0).trunc();
+                (Some(f as i64), abs - f * 12.0)
+            } else {
+                (None, abs)
+            };
+            let whole = in_rem.trunc();
+            let frac = in_rem - whole;
+            let denom = 64u64;
+            let numer = (frac * denom as f64).round() as i64;
+            let mut n = numer as u64;
+            let mut d = denom;
+            while d > 1 && n % 2 == 0 && d % 2 == 0 {
+                n /= 2;
+                d /= 2;
+            }
+            let frac_str = if n == 0 || d == 1 {
+                String::new()
+            } else {
+                format!(" {}/{}", n, d)
+            };
+            let unit_suffix = if ctx.lunits == 4 { "\"" } else { "" };
+            match feet {
+                Some(f) => format!("{}{}'-{:.0}{}{}", sign, f, whole, frac_str, unit_suffix),
+                None => format!("{}{:.0}{}", sign, whole, frac_str),
+            }
+        }
+        _ => format!("{:.*}", prec, value),
+    }
+}
+
+/// Format an angle (input in radians) using AUNITS / AUPREC.
+#[allow(dead_code)]
+pub fn format_angle(value_rad: f64) -> String {
+    let ctx = unit_context();
+    let prec = ctx.auprec.max(0) as usize;
+    match ctx.aunits {
+        1 => {
+            // DMS — degrees / minutes / seconds.
+            let deg = value_rad.to_degrees();
+            let sign = if deg < 0.0 { "-" } else { "" };
+            let a = deg.abs();
+            let d = a.floor();
+            let m_full = (a - d) * 60.0;
+            let m = m_full.floor();
+            let s = (m_full - m) * 60.0;
+            format!("{}{:.0}°{:.0}'{:.*}\"", sign, d, m, prec, s)
+        }
+        2 => {
+            let g = value_rad.to_degrees() / 0.9;
+            format!("{:.*}g", prec, g)
+        }
+        3 => format!("{:.*}r", prec, value_rad),
+        _ => format!("{:.*}°", prec, value_rad.to_degrees()),
+    }
+}
+
 pub fn square_grip(id: usize, world: Vec3) -> GripDef {
     GripDef {
         id,
@@ -33,7 +152,7 @@ pub fn edit_prop(label: &'static str, field: &'static str, value: f64) -> Proper
     Property {
         label: label.into(),
         field,
-        value: PropValue::EditText(format!("{value:.4}")),
+        value: PropValue::EditText(format_length(value)),
     }
 }
 

src/entities/mod.rs

@@ -1,7 +1,7 @@
 pub mod arc;
 pub mod attribute;
 pub mod circle;
-mod common;
+pub mod common;
 pub mod dimension;
 pub mod ellipse;
 pub mod hatch;

src/io/xref.rs

@@ -143,6 +143,13 @@ fn merge_xref_into_block(
 ) {
     let prefix = xref_block_name;
 
+    // Carry the xref's INSUNITS onto the host BlockRecord. Used at INSERT
+    // time so the inserted xref scales to the host's units (INSUNITS).
+    let src_insunits = xref_doc.header.insertion_units;
+    if let Some(br) = doc.block_records.iter_mut().find(|b| b.handle == br_handle) {
+        br.units = src_insunits;
+    }
+
     // ── Layers ──────────────────────────────────────────────────────────
     // Prefix every xref layer (including "0"). Entity layer references are
     // remapped below so the resolver finds the merged copy. Host layers