perf(rdp): gate H.264 decoder lifecycle

[Mydstiny]

Summary

• add evidence-based AVC420/AVC444/direct-texture capability gates
• serialize hardware-decoder callbacks, flush, and destroy with generation leases
• require a valid renderer GL context before hardware decoder initialization
• expose an isolated diagnostic AVC420 decoder probe and H.264 gate reasons
• stop RDP from creating the RustDesk shared decoder; RDP remains on its GDI/RDPGFX-owned pipeline
• preserve the merged remote IME preview-text fix

Safety

• rdpGfxResetPathSafe() remains hard-disabled
• all production H.264 evidence defaults to false
• FreeRDP_GfxH264 therefore remains disabled
• the probe is diagnostic-only and is not invoked by the normal RDP connection path

Validation

• local API 23 documentation checked for AVBuffer, VideoDecoder, and NativeImage APIs
• native tests: 105/105
• default@OhosTestCompileArkTS
• production assembleHap for arm64-v8a and x86_64
• Light open-source compliance gate
• git diff --check
• pre-push Light gate

Pending device validation

• run isolated AVC420 probe on supported phone/2in1 hardware
• verify RustDesk hardware/software decoder fallback and teardown
• verify RDP remains on GDI with H.264/Reset advertisement disabled
• complete non-pressure connect/exit/reconnect checks

This PR remains draft until suitable real hardware is available. Pressure/soak testing is intentionally deferred.