feat(lego): PerpendicularHolesLiftarm — studless liftarm with perpendicular pin holes

[fa-mc]

Summary

Adds a parametric thick studless liftarm where each hole position bores along either the flat-face axis (+Z, "main") or the narrow side-face axis (±Y, "perp") — generalizing the LEGO 6435016 / design-2391 "Liftarm Thick with Perpendicular Holes" family. hole_axes=None defaults to the alternating [perp, main, …] pattern (the reference image's layout).

PerpendicularHolesLiftarm(num_holes=5)                              # default: perp,main,perp,main,perp
PerpendicularHolesLiftarm(num_holes=5, hole_axes=["main"]*5)        # == LegoTechnicBeam(5)
PerpendicularHolesLiftarm(num_holes=3, hole_axes=["perp","main","perp"])

Default 5-hole part: 3 perpendicular (±Y) + 2 main (+Z) counterbored pin holes, single solid, lead-in chamfers on all 10 mouths. Visual contracts: visual_contracts/2026-06-26-perpendicular-holes-liftarm_design_{iso_ne,front}.svg.

Architecture

• New sibling class vibe_cading/lego/technic_beam_perp.PerpendicularHolesLiftarm — not a widening of LegoTechnicBeam's narrow (length_in_studs) contract (dual-lens deletion-test reasoning in the design doc).
• Extracted a shared stadium_beam_body() helper from LegoTechnicBeam — removes body duplication, zero behavior change.
• _HoleMouthSelector gained a purely-additive axis="z"|"y" knob; the existing LegoTechnicBeam and LegoTechnicLLiftarm call sites are unchanged.
• No cross-drilling (one axis per position); single-solid + main ∩ perp == 0 intersection guards.

Design flow

Artifacts under docs/design_plans/2026-06-26-perpendicular-holes-liftarm_* (requirements + design with human gates). Independent fresh-context TL + developer review plus a 6-dimension adversarial verification pass caught two real defects:

1. The first selector design would have broken LegoTechnicLLiftarm (a second consumer with non-zero-Y rims) — reworked to be purely additive.
2. A CI-red engine_api.json omission (falsely reported as done) + a "lying" selector comment (perp rims claimed off-center; they're at mid-height for n≥2) — both fixed and independently re-verified.

Validation

• 41/41 tests (new tests/test_technic_beam_perp.py + NFC on beam & L-liftarm)
• NFC: PerpendicularHolesLiftarm(N, all-main) == LegoTechnicBeam(N) at 0.00% volume delta (boolean-residual checked)
• 18/18 visual contracts fresh; flake8 / no-__main__ clean; engine_api.json deterministic
• Real build.py manifest pipeline builds the registered entry (897 KB STEP)
• Version 0.1.4 → 0.1.5 (public-surface addition)

Reviewer notes

• build.toml entry added (user-approved): lego/perpendicular_holes_liftarm_5hole.step.
• Branch is off main; an unrelated concurrent crossed-helical-mesh_review.md from a parallel session is intentionally not included.

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