refactor(tess): move per-entity tessellation into entities/

Each entity's tessellation now lives alongside its definition in
entities/*.rs, attached to the entity type via a local trait:

  LegacyTess     — Viewport / Insert / Hatch / Ole2Frame
                   (fallback path for entities not on the truck pipeline)
  LeaderTess     — Leader
  MultiLeaderTess — MultiLeader
  DimensionTess  — Dimension (carries its dim-only helpers too:
                   ~1700 lines of dim_geometry, dim_text_*, format_*,
                   tolerance, suppression, etc.)

scene/tessellate.rs keeps only the cross-entity dispatcher plus the
shared dim machinery reused by leader/multileader/dimension
(ArrowKind, DimGeom, append_arrow, arrow_from_block, add_segment,
add_polyline, normalized_or, push_tri, color_or_inherit, entity_z,
offset_snap_pts). Cross-entity arc helpers + aci_to_rgba moved to
scene/tess_util.rs.

scene/tessellate.rs shrinks from ~3300 to 809 lines. Compile clean,
all 9 tests pass.

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

src/scene/tess_util.rs

@@ -0,0 +1,90 @@
+// Shared helpers used by per-entity tessellation impls in `crate::entities`
+// and the dispatcher in `crate::scene::tessellate`.
+//
+// Anything cross-entity (colour resolution, arc sampling, chord tolerance)
+// lives here. Entity-specific helpers (dim format strings, mleader path
+// types, etc.) stay with their entity file.
+
+use acadrust::types::Color as AcadColor;
+use glam::Vec3;
+
+use crate::scene::wire_model::{SnapHint, TangentGeom, WireModel};
+
+/// Output of the legacy per-entity geometry path used by entities not
+/// (yet) covered by the truck topology pipeline (Viewport, Insert,
+/// Hatch outline, Ole2Frame, …). Tuple form preserved to avoid
+/// touching every callsite when the dispatcher wraps these into a
+/// WireModel.
+///
+/// Layout: `(points, snap_pts, tangent_geoms, key_vertices)`.
+pub type LegacyGeometry = (
+    Vec<[f32; 3]>,
+    Vec<(Vec3, SnapHint)>,
+    Vec<TangentGeom>,
+    Vec<[f32; 3]>,
+);
+
+// ── Arc tessellation helpers ─────────────────────────────────────────────
+
+/// Convert hatch-boundary arc `(start, end, ccw)` into a
+/// `(start, signed_span)` ready for the sampling loop. Matches the
+/// legacy `(TAU - sa, TAU - ea)` flip used here for years — direction
+/// semantics are preserved on real files. (Wrap-through-2π is a known
+/// edge case in that convention; do not "fix" it without a wider audit
+/// of how upstream writers emit CW boundary arcs.)
+pub fn arc_signed_span(start: f64, end: f64, ccw: bool) -> (f64, f64) {
+    const TAU: f64 = std::f64::consts::TAU;
+    let (sa, ea) = if ccw { (start, end) } else { (TAU - start, TAU - end) };
+    (sa, ea - sa)
+}
+
+/// Segment count for an arc, targeting `chord_tol_world` chord-height
+/// error in world units. Floor 8, cap 512.
+///
+/// Two production callers:
+/// - hatch fill polygon (built at load / on edit) passes a radius-
+///   relative tol via [`fill_chord_tol`] — ~0.1% radius, zoom-free so
+///   the polygon stays sharp at extreme zoom-in without re-tessellation.
+/// - hatch wire outline (re-tessellated every frame inside the render
+///   scope) passes a zoom-adaptive tol via [`wire_chord_tol`] — pulls
+///   from the per-frame override set by `Scene::wires_for_block` so
+///   far-out arcs collapse to a handful of segments.
+pub fn arc_segments(radius: f64, span_abs: f64, chord_tol_world: f64) -> u32 {
+    if span_abs < 1e-9 || radius < 1e-9 {
+        return 1;
+    }
+    let tol = chord_tol_world.max(1e-9).min(radius * 0.99);
+    // θ where r * (1 - cos(θ/2)) = tol  →  θ = 2 * acos(1 - tol/r).
+    let max_step = (2.0 * (1.0 - tol / radius).acos()).max(1e-6);
+    ((span_abs / max_step).ceil() as u32).clamp(8, 512)
+}
+
+/// Chord tolerance for the load-time fill polygon: 0.1% of radius,
+/// floor 1 µm so degenerate radii still produce a workable count.
+pub fn fill_chord_tol(radius: f64) -> f64 {
+    (radius * 0.001).max(1e-6)
+}
+
+/// Chord tolerance for the per-frame wire outline: pulls the active
+/// `truck_tess::set_curve_tol_override` value (Scene sets it to
+/// `world_per_pixel × 0.5` so curves stay at ~half-pixel chord error at
+/// the current zoom). When no override is active (snap / hit-test
+/// passes, load-time builds), falls back to [`fill_chord_tol`] so we
+/// never under-sample.
+pub fn wire_chord_tol(radius: f64) -> f64 {
+    match crate::scene::truck_tess::active_curve_tol() {
+        Some(t) => t.min(fill_chord_tol(radius)).max(1e-9),
+        None => fill_chord_tol(radius),
+    }
+}
+
+// ── Colour helper ──────────────────────────────────────────────────────────
+
+/// Convert an acadrust Color (ACI index or true-color) to a GPU RGBA value.
+pub fn aci_to_rgba(color: &AcadColor) -> [f32; 4] {
+    if let Some((r, g, b)) = color.rgb() {
+        [r as f32 / 255.0, g as f32 / 255.0, b as f32 / 255.0, 1.0]
+    } else {
+        WireModel::WHITE
+    }
+}