feat(cookbook): index mechanism/assembly snippets for lookup_cookbook

[w1ne]

The gap (KC-10)

lookup_cookbook had no reachable mechanism content. Every one of these returned single-body geometry instead — the top hit for the first was a snippet about mirroring a symmetric part:

• "assembly with parts connectors and mates"
• "hinge"
• "revolute joint"
• "how do I declare a connector"

The cost is bigger than discoverability. A connector origin must use the tagged form:

base.connector('pivot', { type: 'axis', origin: { kind: 'vec3', value: [5, 20, 16] }, axis: [0, 1, 0] });
arm.mate('elbow', 'base.pivot', 'arm.pivot', 'revolute', { limitsDeg: [0, 90] });

A bare [x, y, z] array instead crashes the evaluator with an unhandled Cannot read properties of undefined (reading 'kind') — an internal TypeError, not a validation diagnostic. That tagged form appears in no tool documentation. The only place it was written down was cookbook source that lookup_cookbook could not reach, so the discovery tool couldn't surface the one fact that prevents the crash.

The change

Two snippets in the indexed corpus — a two-link arm (connector + revolute mate) and a clamshell hinge — the six tags that make them findable, and a test asserting all four queries return a mechanism snippet and that the returned body shows the tagged origin form.

Verification

• Both snippets actually execute against the live engine: ok: true, 2 parts, zero diagnostics, for each. A cookbook snippet that doesn't run is worse than no snippet, so this was checked rather than assumed.
• Negative control: with the two snippet files removed, all 5 tests fail; restored, all 5 pass.
• src/agent: 362 passed, 0 failed (run in a checkout with generated assets present).
• tsc --noEmit: clean.

One note for anyone verifying in a fresh worktree: assets/parts/ is gitignored and generated, so fetchPart / findPart / listPartCategories / listPartFamilies fail there for that reason alone. They pass in a normal checkout.

Known limitation

This indexes the root cookbook/ corpus. The worked examples under src/agent/skills/kernelcad-kinematic/cookbook/*.kcad.ts (scissor jack, over-centre latch, swept-collision cases) are still unreachable from lookup_cookbook — worth a follow-up, since that's where the richest mechanism material lives.