[bus 9b] Objects domain: store-owned, routed through run

[HaD0Yun]

Part of #434 (plan v3).

Scope

Move the objects domain onto the store, following the pilot (#447). Line refs are at 793b4a4; after #460 the code lives in src/domains/objects.js and the hierarchy/Inspector panels.

Store-owned state: the scene-history storeRef (:2290) and its transactions (:2371-2395) become the objects slice of the document store. Persistence keeps scene.objects and the IndexedDB assets. The write guard turns on for objects.

UI writers through run (48 sites, App.jsx:15069 and nearby, src/hierarchy-panel.jsx, src/object-gizmo.jsx):

• gesture sessions: gizmo drag, scrub, matte (drafts :2479-2487), each through run.begin / run.commit;
• add/import/spawn (:2536-2800), including asset.import, which the UI doesn't route today;
• delete, rename, group/ungroup.

Semantic commands in src/commands/objects.js: object.add, object.remove, object.rename, object.group, object.ungroup, asset.import, and objects.arrange, which takes the planner at src/studio-agent-commands.js:154 as-is. arrange_objects becomes a thin alias. Bulk delete is one atomic retained entry, so it stays open.

Generic fields: add the object kind to <kind>.set (normalized with normalizeSceneObject from src/scene-objects.js) and the objects slice to inspect_studio document.

Depends on

• [bus 8] Pilot: stage domain store-owned, with generic set/read for stage #447 (pilot) store-owned stage and generic set/read

Files and concurrency

• Touches: src/domains/objects.js (owned by this issue), src/commands/objects.js, src/commands/elements.js (adds the object kind), src/hierarchy-panel.jsx, src/object-gizmo.jsx, test/bus/parity-pending.json, test/bus/verify-object-commands.mjs.
• Domain migration: runs alongside at most two of [bus 9a] Shots and camera domain: store-owned, routed through run #441, [bus 9c] Cast, prompt blocks and waypoints: store-owned, routed through run #442, [bus 9d] Scenes and project domain: store-owned, routed through run #461, [bus 9e] Motion and IK domain: store-owned, routed through run #448 (max 3 at a time). No other issue edits src/domains/objects.js.

Acceptance

Each check is a red test(...) commit that fails, then a green commit that makes it pass.

1. The object rows of the parity matrix, all origins x all checks: pending before -> green after, removed from parity-pending.json.
2. A gizmo drag via run.begin: no receipt before -> one undo entry and one receipt after.
3. object.set {scale: 500} from the agent: no command before -> lands the clamped value after. object.set and patch_elements give identical post-state for every patchable object path.
4. inspect_studio document for objects deep-equals the scene.objects projection: missing before -> equal after.
5. A bulk delete of 3 objects: several entries before -> one entry, undone deep-equal after.
6. A direct store.* write outside a run in dev: allowed before -> throws after.
7. Metrics: (a) about 20 fewer store.* refs, (b) about +48 sites.
8. npm run build exits 0, the existing suites pass unchanged, and the browser smoke tests pass.

PR rules

One PR on feat/issue-445 with (#445) in the title, against main. Attach the red/green commit list and the test output. Do not merge.