feat(snap): Shift hard axis lock during point picks (#312)

Holding Shift while a command asks for a point captures the nearest
polar increment (or ortho axis, UCS-aware) from the rubber-band base
toward the cursor and pins the pick to that ray. Every candidate —
osnap hits included — projects onto the locked direction, so a snap far
off-axis contributes only its along-axis component; the click commits
exactly what the preview shows. The direction stays fixed while Shift
is held and drops the moment it releases. Grip drags get the same lock
from the grip origin.

Guards: engages only when the active command wants a point — never
during entity picks (Shift there is the TRIM/EXTEND swap), tangent
picks, selection gathering or window-corner picks.

Closes #312

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/helpers.rs

@@ -250,6 +250,41 @@ pub(super) fn polar_constrain_near(
     }
 }
 
+/// Hard axis lock (#312): the locked ray's direction — the nearest polar
+/// increment (or ortho axis) from `base` toward `cursor`, in the active UCS
+/// plane. `None` when the cursor sits on the base point.
+pub(super) fn axis_lock_capture(
+    cursor: glam::DVec3,
+    base: glam::DVec3,
+    polar: bool,
+    step_deg: f32,
+    xf: &UcsXform,
+) -> Option<glam::DVec3> {
+    let p = xf.to_ucs(cursor);
+    let b = xf.to_ucs(base);
+    let dx = p.x - b.x;
+    let dy = p.y - b.y;
+    if dx.hypot(dy) < 1e-9 {
+        return None;
+    }
+    let step = (if polar { step_deg as f64 } else { 90.0 }).to_radians();
+    let ang = (dy.atan2(dx) / step).round() * step;
+    let dir_ucs = glam::DVec3::new(ang.cos(), ang.sin(), 0.0);
+    let dir = xf.to_wcs(b + dir_ucs) - xf.to_wcs(b);
+    (dir.length_squared() > 1e-12).then(|| dir.normalize())
+}
+
+/// Project `pt` onto the locked ray through `base` — the hard lock applies to
+/// EVERYTHING, including an osnap hit, so a snap far off-axis contributes only
+/// its along-axis component (#312).
+pub(super) fn axis_lock_apply(
+    pt: glam::DVec3,
+    base: glam::DVec3,
+    dir: glam::DVec3,
+) -> glam::DVec3 {
+    base + dir * (pt - base).dot(dir)
+}
+
 // ── Clipboard / selection helpers ──────────────────────────────────────────
 
 /// Copy/paste anchor: the lower-left corner of the bounding box that encloses