feat: define SceneForge Package v1 and runtime adapter

[ewanqian]

Closes #9

Intent

Define the first reviewable, renderer-neutral SceneForge Package v1 boundary and a minimal runtime adapter API without adding networking, backend services, Needle dependencies, or visual features.

Completed in this slice

• Zod schemas for package metadata, assets, screens, cameras, spawn points, cue sets, and quality profiles.
• Namespaced extensions fields for runtime-specific metadata; no Needle-specific core fields.
• Cross-reference, duplicate-ID, portable-path, and readable validation errors.
• Additive migration from the existing VenueManifest, including legacy metadata preservation.
• Minimal RuntimeAdapter covering scene loading/disposal, cue application, camera selection, media-surface binding, and XR capability reporting.
• Minimal R3F reference adapter skeleton behind an injected renderer port.
• Portable package and runtime conformance fixtures with unit tests.
• Units, coordinates, Blender export notes, directory layout, asset URI, and fallback conventions.

Deferred acceptance criteria

• Projectors, navmesh, audio zones, and interaction schemas.
• Input and XR session-entry adapter APIs.
• Networking adapter APIs and shared room/media-track contracts.
• Needle adapter implementation; this remains part of Spike Needle Engine as a replaceable SceneForge runtime #8.
• Full desktop/mobile/Quest/Vision Pro capability policy and measured device evidence.
• Existing viewer cutover from VenueManifest to SceneForgePackage; this slice keeps migration additive.

Validation

• npm run verify
• 5 test files / 15 tests passed
• Production build passed
• Existing VenueManifest fixtures migrate and validate
• Desktop mouse/orbit path checked — no visual/runtime integration changed
• Mobile responsive layout checked — no UI changed
• WebXR device path checked — no XR session behavior changed

Risk and fallback

This is an additive domain/API slice. The current Stage Viewer continues using VenueManifest, so rollback is removal of the new package/runtime modules. WebGL2 and existing WebXR behavior are unchanged. Runtime-specific data is optional and isolated under dotted extension namespaces.