feat: relative broadcast paths for cross-broadcast catalog renditions

[kixelated]

Summary

Rewritten from scratch against dev (the original main-based implementation predated the moq-mux catalog machinery and the js/watch rework) and now hooked up end to end on the consumer side.

A rendition in a hang catalog can set an optional broadcast field: a path relative to the broadcast that served the catalog (e.g. "../source"), pointing at the broadcast that actually publishes the track. A transcoder can then publish a sidecar catalog that adds new renditions while pointing unchanged ones at the original broadcast, instead of re-publishing those bytes.

Schema / path math

• moq-net: new PathRelative type (trims slashes, drops . segments, preserves ..; serde only, never on the announce/subscribe wire) and Path::resolve with excess-.. clamping. Mirrored in @moq/net as Path.normalizeRelative / Path.resolve so both sides agree byte-for-byte.
• hang / @moq/hang: optional broadcast field on VideoConfig and AudioConfig (skipped when unset, so existing catalogs are byte-identical).

Consumer wiring (the new part)

• moq-mux: new Source type bundling the catalog BroadcastConsumer with optional (OriginConsumer, path) context, built via Source::new(broadcast).with_origin(origin, path). All exporters (fmp4, mkv, ts, flv, h264, h265) now take impl Into<Source>; a bare BroadcastConsumer still compiles via From but fails with a clear Error::MissingOrigin if a catalog actually references another broadcast. The shared ExportSource resolves the reference and fetches the broadcast via OriginConsumer::request_broadcast (announced broadcasts resolve immediately; OriginDynamic handlers serve the rest). Self-references and empty resolutions reuse the catalog broadcast instead of opening a duplicate subscription.
• moq-cli subscribe, moq-srt egress, and moq-hls pass their origin through, so all three honor cross-broadcast renditions.
• @moq/watch: Broadcast.trackBroadcast(effect, rel) resolves the reference against the broadcast name and consumes the resolved path on the same connection, caching one subscription per referenced broadcast (shared across renditions, gated on the same reload/announced logic). Wired into the audio + video decoder and MSE pipelines.

Public API changes

• New: moq_net::{PathRelative, PathRelativeOwned}, moq_net::Path::resolve, moq_mux::Source, moq_mux::Error::MissingOrigin, hang::catalog::{VideoConfig,AudioConfig}::broadcast, @moq/net Path.resolve/Path.normalizeRelative, @moq/hang RelativeBroadcastSchema, @moq/watch Broadcast.trackBroadcast.
• Breaking (hence dev): moq-mux exporter constructors and moq_hls::Broadcaster::new now take impl Into<Source> (source-compatible for callers passing a BroadcastConsumer by value); moq-cli's Subscribe::new likewise; catalog format gains an optional field.

Not covered (follow-ups)

• moq-ffi (and the py/swift/kt/go wrappers) and moq-gst/moq-rtc consume tracks from a bare BroadcastConsumer and don't resolve references yet; they need origin context plumbed the same way.
• The Rust client origin only resolves referenced broadcasts that are announced (or served by a dynamic handler); there's no client-side "subscribe by arbitrary path" fallback like conn.consume(path) in JS.
• Pre-existing on dev: Cargo.lock is stale w.r.t. the nvidia-video-codec-sdk git dep (cargo metadata --locked fails before this PR too); left untouched here.

Test plan

• PathRelative normalization + Path::resolve unit tests (Rust and JS mirrors)
• Catalog serde round-trip: field deserializes, None stays off the wire, empty string normalizes (Rust hang + @moq/hang zod)
• moq_mux::Source unit tests: no-override, missing-origin error, self-reference reuse, override resolving through a live origin
• just check (full Rust + JS suites, clippy, fmt, rustdoc, biome)
• Manual end-to-end: publish room/source, publish a sidecar catalog with broadcast: "../source", verify @moq/watch and moq subscribe pull the track from the source broadcast

(Written by Claude Fable 5)

🤖 Generated with Claude Code

[coderabbitai]

Walkthrough

This change adds an optional per-rendition broadcast reference to audio and video configs and implements path utilities to normalize and resolve relative broadcast strings. Runtime code (Broadcast.trackBroadcast and updated decoder/MSE codepaths) now resolves and caches override broadcasts and falls back to the catalog broadcast when absent or self-referential. Importers populate the new field as None by default. Rust and JS tests exercise normalization, resolution, serialization, and subscription behavior.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title accurately summarizes the main change: relative broadcast paths for cross-broadcast catalog renditions.
Description check	✅ Passed	The description is clearly about the same cross-broadcast rendition and path-resolution changes shown in the diff.

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch catalog-broadcast-override

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