feat(hang): generic base catalog with app-layer extensions

[kixelated]

Summary

Supersedes the very stale #1149. Instead of a bespoke Section<T> registry, this leans on what already shipped: the base catalog becomes minimal and applications extend it with plain language features feeding the generic @moq/json / moq_json helper. No new catalog machinery.

• Base catalog is now just video + audio. App-specific sections are gone from @moq/hang.

• Extensions are composition, not a registry:

  • JS: z.extend(Catalog.RootSchema, { scte35: z.optional(Scte35Schema) }) → hand the schema to @moq/json's Producer/Consumer.
  • Rust: #[serde(flatten)] base: hang::Catalog in the app's own struct → use moq_json::Producer<AppCatalog>.
  • The base catalog has no deny_unknown_fields, so an extended catalog stays readable by a plain hang consumer (it just ignores the extra sections). This is what makes e.g. SCTE-35 addable entirely in the application layer (hang.live) without touching hang.

• Removed app sections (chat, user, preview, location, capabilities) from @moq/hang, and their feature subtrees + library exports from @moq/watch and @moq/publish. These are application concerns and move to hang.live; the deleted code remains in git history for that migration. The built-in <moq-watch>/<moq-publish> elements and demo/web never used these, so the UI is unaffected.

• @moq/publish Broadcast extension hooks so the app layer can reuse it:

  • sections: extra catalog sections merged into the published catalog.json (only one producer may own that track, hence the hook).
  • tracks: handlers for app-defined tracks (e.g. a section's own update track), consulted for otherwise-unknown subscriptions.
  • Watch needs no hook: the app subscribes to catalog.json independently with its own extended schema.

Why not redo #1149 as written

Half of #1149 already shipped differently: the generic "typed JSON over a track with validation, publishing, subscribing" machinery it hand-rolled is now @moq/json / moq_json (snapshot + delta, schema validation). And the Rust hang::Catalog was already trimmed to video/audio. So the registry is redundant; the remaining gap was making the JS base schema open-by-extension and moving the app sections out.

Breaking changes (hence dev)

• @moq/hang: removed ChatSchema, UserSchema, PreviewSchema, LocationSchema/PositionSchema/PeersSchema, CapabilitiesSchema; RootSchema no longer has those fields; removed unused encode/decode/fetch; PRIORITY trimmed to catalog/audio/video.
• @moq/watch / @moq/publish: removed the Chat, Location, User, Preview exports and their implementations.
• Catalog wire shape for a base hang broadcast is unchanged (still { video, audio }); apps that relied on hang shipping chat/location/etc. must define those sections themselves.

Cross-package sync

Change	Paired update
rs/hang catalog	js/hang ✅, doc/concept/layer/hang.md ✅ (new "Extensions" section)
js/hang catalog	js/watch, js/publish ✅

No rs/moq-ffi / wrapper changes: hang::Catalog's shape is unchanged, so FFI consumers are unaffected.

Test plan

• just check — EXIT 0 (Rust clippy + JS tsc + biome + remark + cargo doc/shear/sort)
• JS tests: signals, net, json, watch, token + new js/hang/src/catalog/root.test.ts (extension contract)
• Rust: cargo test -p hang -p moq-mux -p moq-ffi all green, incl. new extension_roundtrip test + the Catalog doctest

🤖 Generated with Claude Code

(Written by Claude)

[kixelated]

Superseded by #1658, which targets main (now that moq-json landed there via #1655) and adds the catalog producer/consumer extension mechanism on top of the same base-catalog gutting. (Written by Claude)