Issues 001+002: defer LPF wd=16 + LPF vertical variants

Per user direction at cycle-4 close: file wd=16 (trend prediction
validation) and vertical variants (column-stride TMU behaviour
unknown) as local issues for future cycles. Progress instead to
CDEF (AV1) for codec breadth.

docs/issues/001 — wd=16 prediction validation. Per cycle 4 lesson 4,
trend says wd=16 likely flips M4 negative. Quick incremental cycle
when revisited.

docs/issues/002 — vertical variants. Different memory access pattern
(column-strided vs row-strided). The load-bearing unknown is
whether the cycle 2 +6.9% mixed gain survives the TMU coalescing
shift. If positive, deployment recipe gains symmetry; if negative,
must split by orientation.

Both issues have acceptance criteria + expected outcomes documented.

Cycle 5 next: CDEF (AV1) — codec-breadth expansion.

No Gitea repo exists for daedalus-fourier yet (project is local-
only). If a tracker is wanted, create the repo and migrate these
.md files. For now they live in-tree as part of the project history.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/issues/001-lpf-wd-16-prediction-validation.md

@@ -0,0 +1,71 @@
+# Issue 001 — VP9 LPF wd=16 cycle (prediction validation)
+
+**Status**: open, not blocking
+**Type**: kernel-cycle (cycle 5 candidate)
+**Predicted verdict**: RED (M4 likely negative, per cycle 4 lesson 4)
+**Priority**: low (incremental; trend prediction)
+**Filed**: 2026-05-18
+
+## Background
+
+Cycle 4 (LPF wd=8) closed PASS with M4 delta +4.1 % vs cycle 2 wd=4's
++6.9 %. The downward trend prompted Phase 9 lesson: "wd=16 would
+probably show further R degradation; M4 may flip negative based on
+the trend line." See `docs/k4_lpf8_phase4_7.md §"Phase 9 lessons"`.
+
+This issue tracks the experiment to validate (or invalidate) that
+prediction.
+
+## What to do
+
+Cycle 5 LPF wd=16, mirroring cycle 4's compact structure:
+
+1. **Phase 3**: build `tests/bench_neon_lpf16.c` modelled on
+   `bench_neon_lpf8.c`. NEON symbol: `ff_vp9_loop_filter_h_16_16_neon`
+   (already in vendored `vp9lpf_neon.S`). Capture M3.
+2. **Phase 4-7**: write `src/v3d_lpf_h_16_16.comp` extending the
+   wd=8 kernel with the wd=16 outer-flat path (`flat8out` test, 14
+   writes per row when both flat8out and flat8in pass). New
+   contract: `dst_stride_u8 ≥ 14` (vs cycle 4's ≥ 6) because the
+   flat8out path writes at `base-7..base+6` (14 contiguous bytes).
+3. **Phase 5 review**: mandatory — wd=16 is not as incremental as
+   wd=8 (much larger conditional logic, new contract bound).
+4. **Phase 7**: measure M2, R; if M4 negative as predicted, document
+   trend confirmation and close kernel as "CPU-only" in deployment
+   recipe.
+
+## Expected outcome (per prediction)
+
+| Quantity | Predicted |
+|---|---|
+| M1 bit-exact | 100 % (same pattern as cycles 2/4) |
+| M3 NEON | ~55 Medge/s (slightly faster than wd=8) |
+| M2 QPU isolation | ~12-15 Medge/s |
+| R isolation | 0.22-0.27 (ORANGE, downward) |
+| M4 mixed vs NEON-4 | -2 % to +1 % (borderline; likely negative) |
+| 30fps margin | still 5×+ (user-facing PASS regardless) |
+
+## Acceptance criteria (issue closed when)
+
+- Cycle 5 phases 1-7 complete, committed
+- `docs/k5_lpf16_phase*.md` produced
+- Phase 7 verdict documented, deployment recipe updated either way
+- Phase 9 lesson 4 trend prediction validated or refuted
+
+## Why deferred (not done in current session)
+
+The session goal was "continue until user intervention necessary."
+User directed: file as issue, progress to cycle 5 CDEF instead.
+The trend prediction is interesting but the project's deployment
+recipe is already locked through cycle 4; cycle 5 wd=16 result
+would update at most one row of the recipe table.
+
+## Related
+
+- `docs/k4_lpf8_phase4_7.md §"Phase 9 lessons"` lesson 4 (the
+  prediction this validates)
+- `external/ffmpeg-snapshot/libavcodec/aarch64/vp9lpf_neon.S`
+  (NEON ref already vendored — symbol `ff_vp9_loop_filter_h_16_16_neon`)
+- `docs/k2_deblock_phase4.md` (cycle 2 template)
+- `docs/k4_lpf8_phase4_7.md` (cycle 4 template, the most direct
+  reference)