feat(v1.0a2): cross-engine behavior runtime — Babylon auto-rotate + bob + conformance

[longyi-xw]

What

v1.0 sub-stage A2 — proves the behavior runtime holds across engines. Formalizes installBehaviors / tickBehaviors / uninstallBehaviors onto IRuntimeAdapter, gives Babylon its own behavior framework (auto-rotate + bob), and validates runtime parity via the shared conformance suite. Headless only.

How (TDD, per-task commits)

1. Babylon behavior framework (src/runtime/babylon/behaviors/) — BabylonBehaviorRegistry + auto-rotate (quaternion increment) + bob, mirroring the Three structure and sharing the engine-neutral BehaviorDefinition.
2. Contract — installBehaviors/tickBehaviors/uninstallBehaviors added to IRuntimeAdapter (ThreeAdapter already satisfied; NoopAdapter stubbed); BabylonAdapter implements them mirroring ThreeAdapter's behaviorRuntime map + error isolation (console.warn+skip for unknown/invalid, console.error+catch on throw); getSupportedBehaviors returns the Babylon registry.
3. Conformance — one shared describeAdapterConformance run against both engines:
   • bob → describeNode().position exactly equals base + amp·sin(2π·f·t) (cross-engine identical formula + handle state)
   • auto-rotate → ran / accumulates / freezes on uninstall (rotation semantics differ across engines — not asserted bit-for-bit, consistent with the A1 gap)
   • disabled bindings skipped; unknown types isolated (no throw)

Spec: docs/superpowers/specs/2026-06-10-v1.0a2-babylon-behaviors-design.md
Plan: docs/superpowers/plans/2026-06-10-v1.0a2-babylon-behaviors-plan.md

Scope (deferred, in roadmap)

generateBehaviorCode for Babylon → A3 (export). hover-highlight (pointer/material) + setBehaviorDomElement on the contract → B (render host / event story). dispose() behavior-handle release: A2 handles have no dispose, so behaviorRuntime.clear() suffices; noted in Backlog for when a stateful Babylon behavior lands.

Verification

• ✅ pnpm lint (0 new) · pnpm typecheck (0) · pnpm test (518/518)
• ✅ Conformance: ThreeAdapter 13/13 + BabylonAdapter 13/13 — same suite, both engines green (incl. 4 behavior cases)
• N/A visual smoke — A2 is headless, by design

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