feat(moq-ffi)!: expose Subscription and TrackInfo instead of hardcoding defaults

[kixelated]

Summary

Follow-up to the accept(None) cleanup: several moq-ffi spots hardcoded a None/default where the underlying moq-net API takes a caller-controllable value, so FFI consumers had no say over delivery or track properties.

• subscribe_track / subscribe_media / TrackProducer.consume used subscribe(None), dropping the whole Subscription (priority, ordering, staleness, group range).
• publish_track used create_track(name, None), dropping TrackInfo (priority, cache, compression, ordering, timescale).

This adds two uniffi::Records mirroring moq-net — MoqSubscription and MoqTrackInfo, each with per-field defaults — and threads them as optional args through the four methods.

uniffi 0.31 supports #[uniffi(default)] on record fields but not function arguments, so the FFI args are required Option<_>; each language wrapper supplies the native default.

Bindings

Each wrapper forwards the new optional param so end users aren't broken:

• py: subscription=None / info=None kwargs; moq.Subscription / moq.TrackInfo re-exported.
• swift: subscription: Subscription? = nil / info: TrackInfo? = nil; typealiases added.
• go: *Subscription / *TrackInfo params (nil = defaults); type aliases added.
• kt: Subscription / TrackInfo typealiases (kt calls the FFI directly).
• docs: no change — the only examples calling these are Python (which default) and the kt/swift dynamic examples use writeFrame/abort.

Caveat (go)

Go has no constructor defaults, so a partially-built Subscription{Priority: 3} gets Go zero values (e.g. ordered=false) rather than the record defaults. Passing nil uses moq-net's real defaults and is the documented path. Happy to add NewSubscription() / NewTrackInfo() helpers if you'd like construction to match the other languages.

Test plan

• Rust: 22 moq-ffi tests pass; clippy + fmt clean via nix.
• Python: full suite (40 tests) passes, including a new one exercising TrackInfo + Subscription end-to-end; ruff + pyright clean.
• gofmt clean on the go edits. swift/kt updated by inspection (no xcframework/gradle locally).

Stacking

Stacked on #1761 (it reuses the MoqTrackProducer Pending/Active refactor from that PR), so the base is claude/happy-jang-9553c6 and the diff shows only this change. Retarget to dev once #1761 lands.

🤖 Generated with Claude Code

(Written by Claude)