build: add opt-in Cargo/cc::Build OBS prototype

[zackees]

Shipping scope

Ship the opt-in Cargo/cc::Build prototype and bundled obs-sys path. The existing CMake build remains the default. This PR does not claim the full CMake replacement, complete platform coverage or release packaging readiness.

The maintainer explicitly requested getting this code merged and doing follow-ups afterward. Additional C/Rust parity expansion is deferred to #1390; the complete native-build replacement remains tracked in #1332. Existing tests and public C headers are preserved.

Included

• Typed native build/link orchestration, real dynamic libobs/plugin contracts, generated configuration, canonical source manifests, input invalidation and bundled obs-sys configuration.
• Linux C/Rust libobs twins sharing common objects, dynamic plugins, OpenGL, Qt frontend/resources, recording/mux helpers, scripting, websocket, browser/CEF and the implemented encoder/capture/audio modules. This is a demonstrated selected-feature application, not a claim that every optional plugin is converted.
• Unity on / compiled PCH off by default, with both optional. Real compiler cache reuse through soldr is measured; restored archives are not counted as compiler hits. PCH audit found18 requests/14 artifacts,14 initial compiler executions and0 on warm repeat;4 redundant cache requests remain a performance follow-up.
• Existing CMake source selection consumes the canonical manifests where converted. No new GitHub workflow files.

Correctness changes included

• HEVC-disabled MP4/MOV/VAAPI guards and codec advertisement, found through real disabled-option recording runs.
• Sub-sample audio-buffer interval initialization, also proposed separately in fix: preserve audio intervals for sub-sample buffering delays #1385 with new RED/GREEN regression coverage.
• Browser startup readiness through the forward obs-browser submodule revision (merged dependency fix: coordinate CEF initialization, browser tasks and unload obs-browser#1, merge b0f67b58; the selected production commit remains reachable).
• Integration harness snapshots its runner before lengthy native builds and honors the GUI's existing implicit recording enablement.

Completed validation

Detailed commands, runtime artifacts, RED/GREEN commits, scope and limitations: docs/rust-port/cc-build.md.

• Native shared core/plugin loading, rebuilt dependency/input invalidation and exact libobs twin export comparison; bundled obs-sys API/lifecycle checks.
• Earlier completed app runs against both cores: staged OBS GUI, rendered browser/source/dock pixels, JavaScript/websocket behavior, saved configuration, frontend/API recordings and normal shutdown.
• Existing selected test graph:52 core executables,6 VLC,1 graphics and2 standalone WHIP executables. Completed runs include104 core executions/832 reported cmocka cases,50 VLC cases,8 group-audio cases and8 standalone WHIP cases. These are separately scoped runs, not a claim of a single final-head full-suite pass.
• WebRTC: all4 unity/PCH combinations each pass2 real H.264/Opus streams and4 recordings against both cores. Receiver validates decoded pixels, stereo48kHz1kHz tone, advancing timestamps, authenticated signaling/DELETE and cleanup. Original CMake module matches26 exports and passes the same separate reference run. Global HEVC-off streaming and recordings also pass.
• Previous all-target/all-feature Clippy and read-only review pass; integration binary cap remains25/25. The latest documentation-scan false positive is fixed in26f33f149 with focused RED and all6 toolchain tests GREEN.

Current shipping checks

Required CI on26f33f149 PASS, including the final CI gate: run39097098398. The prior head passed Fast Gate, Qt, Windows/macOS Rust tests/cross lint/Tier2 and Linux ON, but failed the fixed documentation scan and the existing recording-shutdown race tracked in #1283. The focused regression correction is in #1392 (issue #1391), with deterministic RED/GREEN coverage and unchanged original tests. No tests are skipped/weakened or retried just to report green.

Final-head local shipping checks PASS on26f33f149: all-target/all-feature Clippy; both staged OBS variants with browser/VLC/WebRTC selected; actual browser pixels/JavaScript, websocket/profile/frontend recording and normal shutdown;4 additional validated API recordings;2 real decoded H.264/Opus WHIP streams with timestamps/tone/authenticated cleanup;8 unchanged standalone WHIP cases. The complete native invocation took440.60s including compilation and runtime checks, not a controlled build benchmark. Runtime directories: app-runtime-XGjhaF, app-runtime-7EQ8Pb. Prior failed/interrupted attempts are retained and not counted as passes. Required PR CI passed. This PR is ready and enqueued normally behind #1392; merge-queue validation is running: run39098913076.

Follow-ups

• test: finish Cargo native-build C/Rust parity validation after prototype launch #1390: additional C/Rust parity/test-graph validation; this is explicitly deferred by the maintainer.
• build: replace CMake with Cargo and cc::Build, preserving the complete OBS application #1332: remaining plugins, native Windows/macOS graphs/runtime, physical capture/hardware codec paths, SDK acquisition, standalone publishing, installation/release packaging and graph-wide performance work.
• Actual WebRTC HEVC/AV1/simulcast media, static SDK variants and external services remain unverified; advertised codec support is not runtime proof.

Refs #1332, #1385, #1390. The full conversion remains open under #1332.