feat(agent,server): v2 secure-session-core Task 7 - HW H.264 + negotiated raw fallback

SPEC-002 Phase 1 Task 7 (the last), code-reviewed APPROVED, locally verified
(cargo fmt + clippy -D warnings exit 0 + cargo test --workspace 89 pass + build).

- Encoder trait + factory: RawEncoder (salvaged, UNCHANGED) and H264Encoder,
  selected by negotiation; factory falls back to raw on H.264 init failure.
- Negotiation: agent advertises supports_h264 (MFTEnumEx HW probe, cached) in
  AgentStatus; server picks the codec via select_video_codec(supports, prefer)
  and stamps StartStream.video_codec; agent re-guards on local HW. Policy
  constant DEFAULT_PREFER_H264 = false, so RAW is negotiated for every session
  today - H.264 stays dormant until live hardware validation (Task 8).
- MF H.264 encoder (h264.rs, FIRST-CUT / compile-verified-only): HW encoder MFT,
  BGRA->NV12 (color.rs, unit-tested), sync drain, fall-back-to-raw on any failure.
- Viewer H.264 decoder (decoder.rs, FIRST-CUT): MF decoder on a dedicated COM
  thread; drops+logs on failure, raw render path untouched.
- proto additive: VideoCodec enum, StartStream.video_codec=3,
  SessionResponse.video_codec=5, AgentStatus.supports_h264=11.
- Raw+Zstd path byte-for-byte unchanged; remains the guaranteed default/fallback.

Review confirmed unsafe impl Send for H264Encoder is sound (single-owned &mut on
the block_on thread; session future never spawned) and every MF failure degrades
to raw. H.264 is NOT claimed functional - compile/clippy/build-verified only;
live validation + force-IDR + the no-spawn-invariant doc are Task 8 go-live gates.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

proto/guruconnect.proto

@@ -17,6 +17,23 @@ message SessionResponse {
     string session_id = 2;
     string error = 3;
     DisplayInfo display_info = 4;
+    // Negotiated video codec for this session (Task 7). The plan models the
+    // codec selection here; however the LIVE server->agent handshake in v2 is
+    // done over WebSocket query params + StartStream (SessionRequest/
+    // SessionResponse are not exchanged on the wire today), so the codec the
+    // agent actually applies is carried on StartStream.video_codec below. This
+    // field is kept for spec parity / future use; raw is the safe default.
+    VideoCodec video_codec = 5;
+}
+
+// Negotiated video codec (Task 7). RAW = salvaged BGRA+Zstd+dirty-rects, the
+// guaranteed fallback and current default. H264 = hardware Media Foundation
+// encode, the negotiated upgrade (compile-verified; validated on hardware in
+// plan Task 8). H265/HEVC is future opt-in (TODO).
+enum VideoCodec {
+    VIDEO_CODEC_RAW = 0;    // Raw BGRA + Zstd (safe default / fallback)
+    VIDEO_CODEC_H264 = 1;   // Hardware H.264 via Media Foundation
+    VIDEO_CODEC_H265 = 2;   // Future / opt-in (not implemented)
 }
 
 enum SessionType {
@@ -268,6 +285,13 @@ message Disconnect {
 message StartStream {
     string viewer_id = 1;           // ID of viewer requesting stream
     int32 display_id = 2;           // Which display to stream (0 = primary)
+    // Negotiated codec for this stream (Task 7). The server selects this from
+    // the agent's advertised capability (AgentStatus.supports_h264) and its
+    // policy (DEFAULT_PREFER_H264, currently false so we never ship unvalidated
+    // H.264 as the default). The agent builds its encoder from this value;
+    // VIDEO_CODEC_RAW (0, the default for older servers) keeps the salvaged
+    // raw+Zstd path. On any H.264 init failure the agent falls back to raw.
+    VideoCodec video_codec = 3;
 }
 
 // Server commands agent to stop streaming
@@ -287,6 +311,12 @@ message AgentStatus {
     string organization = 8;            // Company/organization name
     string site = 9;                    // Site/location name
     repeated string tags = 10;          // Tags for categorization
+    // HW-encode capability (Task 7). True when the agent enumerated a HARDWARE
+    // H.264 encoder MFT at startup (MFTEnumEx, MFT_CATEGORY_VIDEO_ENCODER,
+    // MFVideoFormat_H264, hardware flag). The server uses this for codec
+    // negotiation (see StartStream.video_codec). Detected once and cached;
+    // false on non-Windows / no HW encoder / MF unavailable.
+    bool supports_h264 = 11;
 }
 
 // Server commands agent to uninstall itself