feat(mediaplayer): HLS / Low-Latency HLS playback (Windows)

[towneh]

Summary

Adds an HLS / Low-Latency HLS source to the media player, layered on top of the existing OS-codec path.

.m3u8 URLs are handled by a new protocol/basis_hls.c, which is not a demuxer — it parses the M3U8, selects a single rendition, starts near the live edge, and stitches the segments (and, for LL-HLS, the EXT-X-PART partial segments) into one continuous byte stream that the existing MPEG-TS / fragmented-MP4 demuxers consume. A background reader paces delivery to the stream's measured average bitrate so the present path is fed at real time rather than flooded, with enough burst headroom to deliver segment-start keyframes promptly. When the origin advertises EXT-X-SERVER-CONTROL:CAN-BLOCK-RELOAD with parts, it uses blocking _HLS_msn/_HLS_part playlist reloads and rides parts to target roughly PART-HOLD-BACK latency (~5 s); against a plain HLS origin you get its segment-bound latency instead.

Scope: Windows (WinHTTP fetch), clear streams, single rendition. Android/Quest support is planned.

Additive only. The existing RTSP / RTMP / MPEG-TS / fMP4 / HTTP implementations have zero source edits — basis_media_core.c only gains a .m3u8 branch ahead of the plain byte-source path, so every other URL (rtsp / rtmp / .ts / .mp4) takes the identical existing route. The Windows x86_64 DLL is rebuilt to include the new source; RTSP and MPEG-TS are behaviourally unchanged, only recompiled. No C# changes.

• protocol/basis_hls.c / .h — new HLS source (M3U8 parse, segment/part scheduler, paced read-ahead buffer)
• basis_media_core.c — route .m3u8 to the HLS source, feeding basis_ts_run / basis_mp4_run unchanged
• CMakeLists.txt — add basis_hls.c to the portable core
• README.md — document HLS support and the Windows / clear / single-rendition scope

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-C-only change (new protocol/basis_hls.c/.h, an additive .m3u8 branch in basis_media_core.c, a CMakeLists.txt line, the rebuilt Windows x86_64 basis_media_native.dll, and README docs). There is no C# / MonoBehaviour / Unity-API code in this PR, so the C#-oriented required checks above — transform access, Addressables, GetComponent, BasisEventDriver, jobification, property/access style, BasisLocalCameraDriver, BasisDebug logging, scene-wide discovery, hot-path allocations/logging/collection access — are N/A; they're ticked per the "tick N/A boxes too" instruction.

The one hot path that does exist is the native background reader that paces segment/part bytes to the demuxer. It runs entirely in C off the Unity main thread, allocates its read-ahead buffer once at init and reuses it, and is rate-limited to the stream's measured average bitrate (with burst headroom for segment-start keyframes) so the present path is fed at real time rather than flooded.

Tested on Windows against live HLS and LL-HLS origins; the ~5 s latency target requires an LL-HLS origin (plain HLS gives its segment-bound latency). Not exercised on Linux/macOS/Android/iOS — Android/Quest support is planned as a follow-up.