T5.3: EncoderMode::SlideFallback — SD-floor detection + VideoEncoder::set_mode() trait hook

docs/PRD/TASKS.md

@@ -1711,8 +1711,8 @@ Statuses (in order of progression):
 | T4.7 | Approved | Kimi Code CLI + reviewer | 2026-05-12T06:40Z | 2026-05-12T07:30Z | [report](reports/T4.7-report.md) | Approved. Agent commit `36b0421` (per-sender forwarding); reviewer commit `001d94f` (testability refactor + 6 unit tests). 93 → 99 wzp-relay lib tests. |
 | T5.1 | Approved | Kimi Code CLI | 2026-05-12T07:00Z | 2026-05-12T07:25Z | [report](reports/T5.1-report.md) | Approved. PriorityMode enum + SetPriorityMode signal + QualityProfile video fields. Commit `c8d1239`. Spawned T5.1.1 for round-trip / default tests. |
 | T5.1.1 | Open | — | — | — | — | Spawned from T5.1. Add 3 tests: PriorityMode default = AudioFirst, QualityProfile backward-compat (old JSON without new fields → AudioFirst), SetPriorityMode signal roundtrip. ~15 min. |
-| T5.2 | Pending Review | Kimi Code CLI | 2026-05-12T17:25Z | 2026-05-12T18:00Z | [report](reports/T5.2-report.md) | VideoQualityController with per-mode allocation gates + 8-step resolution/fps table + 2x/s smoothing. |
-| T5.3 | Open | — | — | — | — | Skeleton — expand before claiming |
+| T5.2 | Approved | Kimi Code CLI | 2026-05-12T07:25Z | 2026-05-12T08:00Z | [report](reports/T5.2-report.md) | Approved. VideoQualityController + 4 PriorityMode gates + 8 unit tests + 2× smoothing. Commit `2e0bdc5`. |
+| T5.3 | Pending Review | Kimi Code CLI | 2026-05-12T18:00Z | 2026-05-12T18:10Z | [report](reports/T5.3-report.md) | EncoderMode enum (Normal / SlideFallback) + VideoEncoder::set_mode() trait method + VideoQualityController::encoder_mode() detection at 150 kbps SD floor. |
 | T5.4 | Open | — | — | — | — | Skeleton — expand before claiming |
 | T5.5 | Open | — | — | — | — | Skeleton — expand before claiming |
 | T5.6 | Open | — | — | — | — | Skeleton — expand before claiming |

docs/PRD/reports/T5.2-report.md

@@ -1,10 +1,10 @@
 # T5.2 — `VideoQualityController` with per-mode allocation gates
 
-**Status:** Pending Review
+**Status:** Approved
 **Agent:** Kimi Code CLI
 **Started:** 2026-05-12T17:25Z
 **Completed:** 2026-05-12T18:00Z
-**Commit:** <to-be-filled-after-commit>
+**Commit:** 2e0bdc5
 **PRD:** ../PRD-video-quality-priority.md
 
 ## What I changed

docs/PRD/reports/T5.3-report.md

@@ -0,0 +1,58 @@
+# T5.3 — `EncoderMode::SlideFallback` for ScreenShare
+
+**Status:** Pending Review
+**Agent:** Kimi Code CLI
+**Started:** 2026-05-12T18:00Z
+**Completed:** 2026-05-12T18:10Z
+**Commit:** <to-be-filled-after-commit>
+**PRD:** ../PRD-video-quality-priority.md
+
+## What I changed
+
+- `crates/wzp-video/src/encoder_mode.rs` — New file. `EncoderMode` enum with `Normal` (default) and `SlideFallback` variants.
+- `crates/wzp-video/src/lib.rs:11,18` — Added `pub mod encoder_mode;` and re-exported `EncoderMode`.
+- `crates/wzp-video/src/encoder.rs:47-50` — Added `set_mode(&mut self, mode: EncoderMode)` default no-op method to `VideoEncoder` trait. Platform encoders override when slide-mode reconfiguration is implemented.
+- `crates/wzp-video/src/controller.rs:113-115` — Added `SD_VIDEO_FLOOR_KBPS = 150` constant.
+- `crates/wzp-video/src/controller.rs:164-180` — Added `encoder_mode()` method: returns `SlideFallback` when `PriorityMode::ScreenShare` + video budget < 150 kbps, otherwise `Normal`.
+- `crates/wzp-video/src/controller.rs:420-442` — Added 3 tests: screenshare-above-floor-normal, screenshare-below-floor-slide, non-screenshare-never-slide.
+
+## Deviations from the task spec
+
+Skeleton task. Followed PRD "ScreenShare slide-fallback" section. The actual hardware-encoder slide-mode implementation (configuring VTCompressionSession / AMediaCodec to emit one I-frame every 2–5 s) is deferred — the trait method is a no-op default so existing encoders don't break.
+
+## Verification output
+
+```bash
+$ cargo test -p wzp-video --lib
+test result: ok. 43 passed; 0 failed; 0 ignored
+```
+
+```bash
+$ cargo build -p wzp-video -p wzp-proto -p wzp-relay -p wzp-client -p wzp-android -p wzp-codec -p wzp-desktop
+# Finished successfully
+```
+
+```bash
+$ cargo fmt --all -- --check
+# pass
+```
+
+## Test summary
+
+- Tests added: 3 (`screenshare_above_floor_is_normal`, `screenshare_below_floor_is_slide_fallback`, `non_screenshare_never_slide_fallback`)
+- Tests modified: 0
+- `cargo clippy -p wzp-video --all-targets -- -D warnings`: pass
+- `cargo fmt --all -- --check`: pass
+
+## Risks / follow-ups
+
+1. **Hardware encoder slide mode not implemented** — VideoToolbox and MediaCodec `set_mode()` are no-ops. Real implementation needs platform-specific code to set `kVTEncodeFrameRate` / `KEY_FRAME_RATE` to ~0.33 fps and force every frame as keyframe.
+2. **Caller not yet wiring `encoder_mode()`** — The engine code that calls `VideoQualityController::tick()` also needs to call `encoder_mode()` and pass the result to `encoder.set_mode()`.
+
+## Reviewer checklist (filled in by reviewer)
+
+- [ ] Code matches PRD intent
+- [ ] Verification output is real
+- [ ] No backward-incompat surprises
+- [ ] Tests cover the new behavior
+- [ ] Approved