Runtime bitrate endpoint + ABR capability negotiation (Foundation/Moonlight V+ compatible)

[AsafMah]

Implements #272 — runtime (mid-stream) bitrate changes so Foundation / Moonlight V+ clients can adjust encoder bitrate without reconnecting.

Endpoints (HTTPS, paired client, View permission)

• GET /bitrate?bitrate=<kbps> — apply a new encoder bitrate to the caller's active session. Clamped to max_bitrate and an absolute 500 Mbps ceiling. Returns <root status_code=200><bitrate>N</bitrate></root> (0 = failure) — the exact shape NvHTTP.setBitrate() parses (and its verifyResponseStatus requires the status_code attribute).
• GET /api/abr/capabilities — returns {"supported":false,"version":1,"features":["runtime_bitrate"]}. Verified against AdaptiveBitrateService.kt: supported:false makes V+ run its local ABR controller, which drives the host through /bitrate. So both the manual slider and adaptive mode work. Server-side ABR decisioning (POST /api/abr, /feedback) is intentionally out of scope.

Apply mechanism

A per-session mail event (dynamic_bitrate), coalesced to the latest value on the encode thread:

• NVENC reconfigures the live encoder via nvEncReconfigureEncoder using saved init params (ported from foundation-sunshine, adapted to Vibepollo's runtime-versioned NVENC structs / api::reconfigure_params_version). Seamless, no hitch.
• avcodec encoders (AMD/Intel/software) report set_bitrate()==false and fall back to an encode-session rebuild via the existing reinit path. Correct, but a brief hitch per change. Backend-native AMF/QSV reconfigure is a possible follow-up (mirrors foundation staging NVENC first, AMF later).

Clamping/overflow: rejects <=0, clamps to max_bitrate (when set) and an absolute ceiling; set_bitrate() also rejects >800000 kbps so kbps*1000 cannot overflow the 32-bit rate-control fields.

Files

globals.h (event), stream.{h,cpp} (set_bitrate_for_sessions, per-session event), video.{h,cpp} (encode_session_t::set_bitrate, NVENC override, both encode paths), nvenc/nvenc_base.{h,cpp} (set_bitrate live reconfigure), nvhttp.cpp (routes).

Verification

• Builds + links: compiled with gcc 16 (MSYS2 UCRT64), ninja sunshine → sunshine.exe links cleanly (no undefined symbols from the new functions). NVENC reconfigure struct fields/version confirmed against the bundled ffnvcodec/nvEncodeAPI.h.
• Client contract: response shapes verified against Moonlight V+ NvHTTP/AdaptiveBitrateService source.
• Not yet runtime-tested on a live GPU stream. I don't have a suitable host+client loop to exercise an actual encode session. Requesting a maintainer/community runtime check on NVENC (seamless) and one avcodec backend (reinit fallback). Happy to iterate.

Client reference: qiin2333/moonlight-vplus (NvHTTP.setBitrate, getAbrCapabilities, AdaptiveBitrateService). Server reference: AlkaidLab/foundation-sunshine (#193 /bitrate + NVENC reconfigure, #571 /api/abr/*, ClassicOldSong#690 max_bitrate clamp).