feat(core): a mechanism capability for Play/Stop and E on any kind (AK-09 1/2)

[Aymericr]

AK-09 first slice, part 1 of 2. Part 2 is #955 (GLB clip listing, baked viewer, walkthrough), stacked on this branch and targeting main.

What and why

Play/Stop and E were wired to node.type === 'item' | 'procedural-item' (nodes/src/shared/item-interactions.ts), so a plugin kind with moving parts (Articraft's joints) could not get either.

• Core: capabilities.mechanism: { has, isOn, set, verb? } (MechanismCapability), plus nodeMechanism(node), toggleMechanism(mechanism, node) and toggleNodeMechanism(node). Operating state is transient: set writes useInteractive, never the node. Kinds with one switch use the new useInteractive.mechanisms map (setMechanism, removeMechanism). InteractiveState is exported.
• Action menu: the Play/Stop button is generic (registry-action-contributions.tsx): the single selected node gets it when its kind declares the capability and has(node). The item/procedural contribution keeps only the light switch, so they show the same two buttons, in the same order, with the same classes.
• E: runNodeInteraction(node): the kind's own keyboardActions.e, else its mechanism, else the door/window fallbacks as before.
• Item and procedural-item declare the capability with their existing semantics (itemMechanism, proceduralMechanism). Their E still does mechanisms, else lights. Lights keep their kind checks; a lights capability is the next slice.
• Wiki: node-definitions.md gains capabilities.mechanism.

Visible only once a plugin adopts it. The owner preview ships with the Articraft adoption (AK-ART).

Failing case first

690041dc4 (tests only), on main's code: core mechanism.test.ts 0 pass / 1 fail (no capability); nodes item-interactions.test.ts 0/1 (item and procedural declare none); editor use-keyboard.test.ts + registry-action-contributions.test.tsx fail (E does nothing for a capability-only kind; there is no Play for it).

Proof

• bun run --cwd packages/core test: 2983 pass, 0 fail.
• bun run --cwd packages/nodes test: 3506 tests, 0 fail. The item/procedural parity tests (item-interactions.test.ts, item/definition.test.ts, procedural-item/definition.test.ts) assert the same control values and part states as before.
• bun run --cwd packages/editor test: 1117 pass, 0 fail.
• bun run check: no errors (6 infos, all pre-existing). bun run check-types: every package passes. apps/editor fails on the missing local @webxr/plugin, as on main.
• private-editor bun typecheck against these dists: 12/12.

Rule	Check
R1	API v1 unchanged; the capability is optional. A kind without it behaves exactly as before.
R2	Play/Stop writes only transient useInteractive state: no scene write, no history step (test: running a mechanism is transient).
R6	Operating state stays out of the node; nothing persisted changes.
R8	No agent surface in this slice. MCP creation of plugin kinds is still parked with A-07 flag-on.

R3–R5, R7 and R9 are unaffected: no references, rooms, geometry or recipes change.

House effect: no change. /next has no plugin mechanisms, and item/procedural behave as before.

For Wassim (procedural items, #930)

• nodes/src/shared/item-interactions.ts: proceduralMechanism wraps setProceduralParts(node.id, motionPartIds(node), on), with the same "any running → stop all, else start all" rule, now in toggleMechanism.
• procedural-item/definition.ts: capabilities.mechanism: proceduralMechanism. E runs toggleMechanism(proceduralMechanism, n), else toggleItemLights(n).
• shared/item-interaction-actions.tsx: the Play button moved to the editor's generic MechanismButton, with the same markup and order. The light button is unchanged.

🤖 Generated with Claude Code

[pascal]

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