fix(mediaplayer): render fMP4/CMAF HLS by interleaving fragment audio and video

[towneh]

Summary

HLS with fragmented-MP4 / CMAF segments (EXT-X-MAP:init.mp4 + .m4s) — which includes Twitch's low-latency streams — loaded a single video frame and then stalled with no audio. TS-segment HLS was unaffected.

The demux itself was fine; the problem was emission order. The fMP4 demuxer consumed each moof/mdat track-major: every video sample of the fragment, then every audio sample. With delivery pacing on (always, for HLS) the paced video burst spans the whole fragment duration in wall time, so the fragment's audio always arrived a full fragment late. The decoder trims audio that stale, and its audio-gated presentation clock never got a buffer to start from — one frame, silence.

consume_mdat now emits the moof's trun runs as a decode-time-ordered merge, interleaving the tracks at frame granularity — the same shape the MPEG-TS demuxer's output naturally has. Decode order within each track is preserved, and single-track fragments (split-stream fMP4 legs) are delivered exactly as before.

Both binaries are rebuilt and committed (win-x64 DLL, arm64-v8a .so).

Verification

• Offline A/B on ffmpeg-generated CMAF (H.264+AAC, 2 s fragments, VOD), feeding basis_mp4_run directly and measuring the worst delivery-order PTS gap between tracks: ~2.1 s before (the fragment duration) → ~190 ms after (B-frame reorder depth), with identical access-unit counts and timestamps. A video-only fMP4 stream demuxes identically before and after.
• In-editor: the same CMAF fixture served as HLS VOD over HTTP plays end to end — video renders and the audio tone is audible; auto-detects as on-demand and plays once. Pre-fix behaviour on the identical stream was the single-frame stall.

Scope note

HLS playback remains single-rendition: masters that put audio in a separate EXT-X-MEDIA rendition group (rather than muxing it into the variant) play the selected variant only. This PR fixes muxed CMAF, which is what Twitch low-latency and ffmpeg-style CMAF origins serve.

Required checks

All boxes below must be ticked before this PR can merge. If a check is genuinely N/A, tick it anyway and explain under Notes.

• Tested — I built and ran this locally. The change works in the editor and (where relevant) in a built player.
• Transform access is combined and limited — In hot paths, transform reads/writes go through TransformAccessArray or are otherwise batched. I have not added per-frame transform.position / transform.rotation / transform.localPosition calls inside loops. Whenever I need both position and rotation, I use the combined APIs — SetPositionAndRotation / SetLocalPositionAndRotation for writes, GetPositionAndRotation / GetLocalPositionAndRotation for reads — instead of two separate property accesses; the combined call does one local-to-world matrix traversal instead of two.
• Addressables used for asset/memory loading — Any new asset loads go through Addressables. No new Resources.Load, no direct asset references that pull large content into memory on scene load.
• No new GetComponent / AddComponent where avoidable — Where unavoidable, the result is cached on a field, and any GetComponent<T> is replaced with TryGetComponent<T>(out var x) — bare GetComponent will be denied. TryGetComponent is the modern API (Unity 2019.2+) and skips the Editor-only GC allocation GetComponent causes when a component is missing: Unity wraps the null return in a managed "fake null" object so its overloaded == operator can still detect destroyed C++ objects, and constructing that wrapper allocates; TryGetComponent returns a bool plus out parameter and never builds the wrapper. None of these calls run inside Update, LateUpdate, FixedUpdate, jobs, or other per-frame code paths.
• Per-frame work is scheduled through BasisEventDriver — Any new per-frame work hooks into BasisEventDriver rather than adding standalone Update / LateUpdate / FixedUpdate callbacks on a MonoBehaviour.
• Anything added to BasisEventDriver is bulletproof, or guarded by try/catch — BasisEventDriver runs the single per-frame tick that drives the whole framework (network apply, local player sim, blendshapes, JigglePhysics, nameplates, and more) as one sequential chain. An unhandled exception anywhere in that chain aborts the rest of the tick, so every step after the throwing one is silently skipped for that frame. New work added to the driver must either be guaranteed not to throw, or be wrapped in a try/catch that contains the failure and surfaces it through BasisDebug — logged once / rate-limited, never every frame (see the existing HVRBasisBuiltInAddresses.Simulate() guard for the pattern). Expect this to be scrutinized closely in review.
• Considered jobification — I asked whether this work can be moved to a Unity Job (Burst-compiled where possible). If it can, it is. If it cannot, the reason is in Notes.
• No needless { get; set; } properties or access lockdowns — Public fields are fine; Basis is meant to be read and modified freely, so don't wall things off private/internal without a real reason. Don't wrap a field in { get; set; } when the accessors do nothing — property accessors have a real performance cost vs direct field access, and the lead maintainer prefers plain fields (or a method / setter-only property when only the setter needs logic) over a noop-getter pair. For .Instance singletons, callers reassigning Type.Instance is allowed; if that would break your code, log a warning or throw — don't block the assignment. Locking down access is not your call.
• Camera access goes through BasisLocalCameraDriver — Code that needs the local camera (transform, projection, rig data, etc.) pulls it from BasisLocalCameraDriver rather than looking one up itself. Don't roll a separate camera discovery path.
• Logging uses BasisDebug — All new logging calls go through BasisDebug.Log / BasisDebug.LogWarning / BasisDebug.LogError (with an appropriate LogTag) instead of UnityEngine.Debug.Log / Debug.LogWarning / Debug.LogError. BasisDebug routes through Basis's tagged, color-coded logger and respects the project-wide LoggingDisabled toggle so logging can be killed at runtime; bare Debug.Log calls bypass that and will be denied.
• No scene-wide discovery for dependencies — New code is architected so it does not need FindObjectOfType / FindObjectsOfType / GameObject.Find / FindGameObjectsWithTag to locate what it depends on. References are wired in — registered through an existing manager/driver, injected at init, or passed in by the caller — rather than discovered by scanning the scene at runtime. If a scene scan is genuinely unavoidable, justify it under Notes.
• No allocations in hot paths — Per-frame code (Update / LateUpdate / FixedUpdate, simulation loops, jobs, anything called once per frame or more) does not allocate. No new on reference types, no LINQ, no string concatenation/interpolation, no boxing, no foreach over interface-typed collections. Allocate once at init and reuse the buffer.
• No debugging in hot paths — No log calls of any kind on per-frame paths, including BasisDebug. Hot-path logging floods the console and incurs cost on every frame regardless of whether the message is filtered out downstream. If a hot-path log is needed while iterating, gate it behind #if UNITY_EDITOR and remove (or leave gated) before merge.
• Hot-path collection access is optimized — Cache .Count (lists) / .Length (arrays) into a local int before the loop instead of re-reading the property each iteration. Prefer T[] (with a separate length int when the array is over-sized) over List<T> where the data is hot — Unity's mono BCL doesn't expose CollectionsMarshal.AsSpan(List<T>), so a list can't be fed into Span<T> / unsafe paths cleanly. Where the perf justifies it, drop into Span<T> / ref locals / Unsafe.As / unsafe pointer code to skip bounds checks and copies, and call out the invariants you're relying on under Notes so reviewers can sanity-check them.

Testing details

Tick the platforms you actually tested on. Leave the rest unticked — these are informational and do not block merge.

• Windows
• Linux
• Android
• iOS
• macOS

Input / control mode coverage:

• Tested in VR (note headset under Notes)
• Tested in desktop / non-VR mode
• Tested with phone controls (mobile touch input)
• N/A — change does not touch player/XR/input code

Where applicable, confirm these flows still work after your changes:

• Hot-switching (desktop ↔ VR mode swap at runtime)
• Avatar swapping
• Server swapping (joining / leaving / changing servers)
• N/A — change does not touch any of the above

Notes

• This is a native-plugin change (one function in Native~/protocol/basis_mp4.c); no C# or Unity-side code is touched, so the Unity-specific checks (transforms, Addressables, GetComponent, BasisEventDriver, camera, BasisDebug, scene discovery) are N/A-ticked. Jobification is N/A for the same reason — the work runs on the plugin's demux thread. The demux path allocates per access unit by design (matching the existing code); it is not on a render-frame path.
• The Android .so is rebuilt from the same source. The fix applies there once HLS is enabled on Android (separate branch in flight); nothing on Android regresses meanwhile since the fMP4-over-HLS path can't currently be reached on it.

[towneh]

The media player now lives in its own repository, so this fix moves there — re-created as BasisVR/BasisMediaPlayer#1 with the same commits (paths rebased onto the standalone repo). Closing this one.