docs: add object placement authoring plan

[silbaram]

Motivation

• Provide a focused planning document that defines the enablers and implementation slices required to support a "person places spatial instances; agent attaches behaviors" workflow for Data Scene authoring.
• Clarify ownership boundaries (UI vs agent), the instance.id contract, and the minimal engine work (generic spawn + id↔entity mapping) needed before any dev-only placement overlay work proceeds.

Description

• Add docs/planning/object-placement-authoring-plan.md which documents the problem statement, current diagnostics, the person↔agent ownership model, the instance.id contract, a 0..6 slice implementation plan, verification commands, exclusions, open questions, and next-step recommendations.
• Update docs/planning/README.md to expose the new planning document in the planning index.
• The plan emphasizes a small set of engine enablers (generic data-scene spawn resolver and persistent instance.id↔Entity mapping) and explicitly gates any visual-editor style work behind approval and a dev-only prototype path.

Testing

• Ran pnpm validate:docs-links, which completed successfully and reported docs/link validation OK.
• Ran pnpm build:pages, which failed because examples/starter-runtime/dist was missing and the pages build script requires running pnpm build first.
• Ran pnpm build, which failed during the wasm build step because wasm-pack is not available in the environment; the build:wasm step therefore exited with wasm-pack: command not found.
• Confirmed the environment lacks wasm-pack by invoking wasm-pack --version, which is not present; re-running the full site build will succeed once the wasm toolchain dependency (wasm-pack) and example build artifacts are available.

---

Codex Task