Make video stream optional in RTSP sessions

[steelbrain]

Summary

Removes the assumption that every RTSP session must carry an H.264/H.265 video stream. A session is now valid when any of video, audio, or analytics metadata is set up, so audio-only and metadata-only
configurations (e.g. Axis cameras with video=0, video=0&audio=0&event=on) work end-to-end.

Breaking changes

SessionDescription now groups video and audio fields into VideoStream? and AudioStream? substructs. One Optional check unlocks all the codec-and-parameter fields together — replaces the prior anti-pattern of
multiple parallel Optionals sharing an "all-set-or-all-nil" invariant.

// before
let videoCodec: VideoCodec
let sps: Data
let pps: Data
let vps: Data?
let resolution: (width: Int, height: Int)?
let clockRate: UInt32
let audioCodec: PublicAudioCodec?
let audioSampleRate: UInt32?
let audioChannels: UInt16?
let audioExtraData: Data?

// after
let video: VideoStream?
let audio: AudioStream?

struct VideoStream {
  let codec: VideoCodec
  let clockRate: UInt32
  let sps: Data?           // nil until parameters observed
  let pps: Data?
  let vps: Data?
  let resolution: (width: Int, height: Int)?
}

struct AudioStream {
  let codec: PublicAudioCodec
  let sampleRate: UInt32
  let channels: UInt16?
  let extraData: Data?     // e.g. AudioSpecificConfig for AAC
}

Consumer usage:

if let video = desc.video {
    setupDecoder(codec: video.codec, sps: video.sps, pps: video.pps, vps: video.vps)
}
if let audio = desc.audio {
    audioPlayer.start(codec: audio.codec, sampleRate: audio.sampleRate)
}

The doc comment on SessionDescription documents the invariant: at least one of video, audio, or metadataEncoding is non-nil.

What changed

RTSPSession.start()

• Video find is now optional (no throw on missing video).

• Video SETUP, depacketizer init, and Timeline init are conditional on a successful video SETUP — mirroring the existing best-effort pattern used by audio and metadata.

• New gate before PLAY throws sessionSetupFailed if no supported video, audio, or metadata stream was set up. The error message includes the list of offered streams (e.g. "... (offered: video/jpeg, audio/opus)") so misconfigurations are diagnosable.

• Audio depacketizer init failures now null out audioStreamIndex and friends, matching the metadata-init failure path. Keeps state coherent for the new gate (an audio-only session whose AAC fmtp is malformed
  no longer slips past with audio: nil and zero deliverable streams).

• The empty-Data() fallback for SPS/PPS is gone — video.sps/video.pps are nil until parameters are observed rather than empty Data. Matches the new optional contract; previously a footgun if a consumer
  fed the empty buffer into CMVideoFormatDescriptionCreateFromH264ParameterSets.

• The depacketizer / videoClockRate invariant ("non-nil together") is now expressed as if let depkt = depacketizer, let clockRate = videoClockRate at the construction site — no sentinel fallback needed.

• Decoder setup wrapped in if let video = desc.video, let sps = video.sps, let pps = video.pps.

• Audio setup wrapped in if let audio = desc.audio.

• Video case in the for try await item in session.frames() loop skips when desc.video is nil.

• Window title falls back to "no video" when there's no video stream.

Tests (+3, total 107)

• Three Axis-style SDP fixtures: audio-only, metadata-only, and audio + metadata.
• One DescribeParser test per fixture asserting the expected stream layout, including the negative invariant that no video stream is present.

Docs

• README.md — adds a bullet that audio-only / metadata-only sessions are supported, and an if let video = desc.video snippet in the usage example.
• API.md — replaces the flat SessionDescription block with VideoStream / AudioStream / SessionDescription. Also fills two pre-existing gaps that the metadata PR (Add ONVIF analytics metadata stream support #5) didn't catch: adds
  metadataEncoding, the .metadata case to PublicCodecItem, and a PublicMetadataFrame definition.
• CHANGELOG.md — adds Upcoming entries for both the metadata stream support (also missing from Add ONVIF analytics metadata stream support #5) and this breaking change.

Design choices

• Substruct over parallel Optionals. Five fields with a shared "either-all-set-or-all-nil" invariant is worse than one Optional carrying a substruct: the compiler enforces the invariant once, consumers get one
  unwrap, and codec/clockRate are now non-Optional inside the substruct (they're guaranteed when the stream exists). SPS/PPS/VPS/resolution remain Optional within VideoStream because they reflect "parameters
  observed yet?" — a real semantic distinct from "is there a video stream?".
• Audio gets the same treatment. The flat audio fields had the identical anti-pattern (audioCodec / audioSampleRate / audioChannels / audioExtraData were already parallel Optionals). Folding them into
  AudioStream is cheap when you're already breaking the API.
• Resolution stays as a tuple. (width: Int, height: Int)? — not promoted to a named struct, since that's a separate concern.
• Video SETUP failure is still fatal (matches audio). Reported case is "no video advertised at all," not "video advertised but SETUP rejected." Keeping SETUP fatal is the narrower change; making it
  best-effort like metadata is a defensible follow-up but out of scope here.
• At-least-one-stream check sits before PLAY, not after. A degenerate session has nothing to deliver, and sending PLAY just opens the door to packets we can't route.

Compatibility

• Cameras that advertise video continue to behave exactly as before. The dispatch loop's if let videoIdx = videoStreamIndex guards (pre-existing) already tolerated nil.
• Audio and metadata paths were already optional/best-effort and are unchanged in behavior, with the one symmetric tweak above (audio depacketizer init failure now nulls state).
• Field renames are the only source breakage — desc.videoCodec → desc.video?.codec, desc.audioSampleRate → desc.audio?.sampleRate, etc. No type-system changes beyond grouping.

Test plan

• swift build — clean.
• swift test — 107 tests pass across 16 suites (104 inherited from main + 3 new).
• xcrun swift-format lint --strict -r Sources/ Tests/ Examples/ — clean.
• Example app (CameraViewer) builds with the new substruct flow.
• Pending live test: verify against a real Axis camera with video=0 / video=0&audio=0&event=on before tagging a release. The DescribeParser tests cover SDP parsing; the runtime path is not yet
  exercised against a live audio-only or metadata-only session.