feat: add staged RTSP HEVC input bridge

[anvsk]

Summary

• make NVDEC codec-selectable for H.264 and HEVC Main 8-bit 4:2:0
• select the decoder from RTSP SDP while keeping SRT/TS and traditional RTMP H.264-only
• recover H.264 IDR / HEVC IRAP from Annex-B NAL types when an RTP relay drops the random-access marker
• describe the staged boundary, market rationale, evidence, and remaining V3-04C gate

Verification

• cargo check --locked -p aimedia-rtsp -p aimedia-graph -p aimedia-nvidia
• Linux x86_64 release build with Video Codec SDK 13.0 and header fingerprint 613e2cd436d4d7fbc283e5d92184e7d7f8739ec680f1ee372d580eb801df9ef2
• real RTX 5060 NVDEC proof for one 1080p HEVC Main Annex-B IRAP into the shared NV12 surface contract
• tools/rtsp.ps1 PowerShell syntax validation

Honest status

V3-04A/B/D are complete. V3-04C remains open: two 90-second RTSP software-source runs exposed a missing random-access marker fallback and an empty NVDEC display-queue warm-up bug. Both are fixed here, but the corrected end-to-end gate was not rerun because long/repeated testing was stopped at the user's request. HEVC therefore remains foundation; RTSP remains experimental.

apps/ is intentionally excluded.