fix: harden threads, sketch constraints, and modeling history

[jackControls]

Summary

• implement ISO metric thread tolerance envelopes with recommended internal 6H and external 6g defaults, plus continuous analytic helical OCCT geometry for internal and external modeled threads
• harden sketch snapping, inference, dimensions, midpoint/curve constraints, fillet profile reconstruction, and short-line workflows
• make feature-history insertion, multi-selection/deletion, hole placement previews, and native viewport representations consistent
• stage the OCCT Boolean and healing libraries required by local macOS app bundles

Why

Follow-up testing after #44 exposed several production edge cases:

• coarse grid snapping could outrank exact geometric inference
• consumed sketch edges and midpoint constraints could lose their intended analytic relationship
• modeled and cosmetic thread representations could drift, produce incomplete geometry, or use inconsistent tolerance dimensions
• new operations did not always respect the build cursor or multi-selection state
• local macOS bundles did not directly stage every newly required native OCCT library

This change keeps exact geometry and feature history authoritative while making preview-only rendering follow the owning model feature.

User impact

• more reliable small and constrained sketches, including 0.5 mm segments, intersections, midpoint-on-edge behavior, and editable dimensions
• faster and more robust internal/external thread generation with ISO tolerance-class dimensions
• correct feature insertion at the history marker, improved batch history actions, and steadier hole/thread previews
• portable local macOS builds with the native Boolean/healing dependencies included

Validation

• cargo fmt --all -- --check
• git diff --check
• cargo test --workspace — 395 passed, 2 ignored
• native OCCT test suite — 22/22 passed
• internal and external thread browser end-to-end checks
• production frontend build
• macOS native OCCT app build, signature/portability audit, and launch smoke test

[jeffglousher]

Looks good. Right follow-up to #44 — ISO 6H/6g envelopes match the published limits, external thread stays on an analytic cylinder, and history/preview stay consistent with the owning feature. Approved.