sketch_rect_model: guard against off-plane model_origin (PR #56 follow-up)

[YLZha]

Summary

Implements the helper guard both reviewers of #56 recommended. sketch_rect_model calls modelToSketchSpace to project model_origin onto the sketch plane, then silently dropped the out-of-plane component (sk_o.z) — so an off-plane origin created the body IN the plane, not at model_origin. This was the root cause of 3 of the 4 defects in #56 (back-panel-at-front).

The guard converts that silent wrong-position defect into an immediate, self-explaining error:

sk_o = sk.modelToSketchSpace(Point3D.create(ox, oy, oz))
if abs(sk_o.z) > 1e-4:
    raise ValueError(f"sketch_rect_model('{name}'): model_origin lies {sk_o.z:.4f} cm "
                     f"off the sketch plane — ... sketch on an off_plane at that offset ...")

A correct call (plane at the right offset) has sk_o.z ≈ 0 and is unaffected — only a wrong plane trips it.

Validation

• ✅ cabinet (heavy multi-plane user, including the just-fixed back panel) builds clean — guard does not trip.
• ⏳ Broader example sweep pending — Fusion was contended by a concurrent session when I finished, so I couldn't sweep all examples locally. Relying on CI / a follow-up sweep to confirm no currently-green example trips it. By construction it should be safe: a green example has correct geometry → on-plane origins → no trip. Any trip indicates a real latent off-plane bug (which is exactly what the guard is meant to surface).

Risk note

This is a raise in a core helper used by ~every example. If a currently-green example trips, that's a latent position bug to fix (not a guard false-positive — correct calls can't trip). Do not merge until the example sweep is green (CI or manual).

🤖 Generated with Claude Code

[YLZha]

Validation complete — guard is safe, found + fixed 1 real bug

Guard live-validated on 6 diverse examples (build clean, no trip)

cabinet, wardrobe, bench, crib, sideboard (after fix), bookshelf — covering bare-construction-plane and offset-plane usage, origin-mode and anchored-mode calls.

Static sweep of ALL examples for the bug pattern

A parser over every sketch_rect_model call (bare construction plane + nonzero out-of-plane origin) found sideboard is the only deps-passing example with the origin-mode off-plane bug. The other flagged calls (wardrobe kick, toy-box back, sideboard top) all use anchored mode, which builds from explicit model corners and handles off-plane origins correctly (wardrobe verified clean live).

The bug the guard caught → fixed in this PR

sideboard's kick was built flush instead of recessed. KickFront_Sk/KickLeft_Sk sketched on the bare xZ/yZ planes (y=0 / x=0) while their model_origin set the recess (kick_inset) — silently flattened to flush. The reanchor only fixes the in-plane x/z dims, so the defect survived (no interference → never caught). Fixed by sketching on off_planes at kick_inset (same pattern as #56).

Live-validated: sideboard now builds clean, guard doesn't trip, 0 real interference, and the recessed kick frame is properly connected (all 4 kick boards → Bottom + to each other). Doors/drawer remain separate clusters by design (hinged/sliding).

Ready to merge

The one example the guard tripped is fixed and validated; the static sweep + 6 live builds confirm no other regression. CI will cover the rest.