Issue 003 closed: mixed-kernel M4 validates V4 deployment shape

bench_concurrent_mixed runs NEON-N on kernel A + QPU on kernel B
concurrently. Matrix on hertz:

  V3 (CPU MC + QPU MC same-kernel): CPU 22.64 + QPU 0.39 Mblock/s
  V4 (CPU MC + QPU LPF4):            CPU 27.87 + QPU 12.74 Medge/s
  V1 (CPU MC + NEON-fb CDEF):        CPU 24.49 + 1.75 Mblock/s CDEF
  V2 (CPU LPF4 + NEON-fb CDEF):      CPU 27.28 Medge + 1.70 Mblock/s

V4 is the daedalus-fourier deployment shape (CPU runs MC; QPU runs
LPF4 via cycle 2 GREEN offload). Both substrates productive; CPU
MC +23% per-core vs same-kernel V3 control. Same-kernel M4 in
cycles 1-5 was a worst-case contention bound, not a deployment
number — user's "5%/50%" framing was correct.

Cycle 3 MC verdict unchanged (QPU MC contributes ~0.4 under any
contention); cycle 5 CDEF deferred verdict softened to
opportunistic helper (NEON-fallback proxy used since cycle 5
Phase 6 not yet built).

- tests/bench_concurrent_mixed.c (configurable cpu-kernel /
  qpu-kernel matrix; supports MC, LPF4, LPF8, IDCT real QPU
  dispatch; CDEF uses NEON-on-core-3 fallback)
- CMakeLists.txt: build target wired with all FFmpeg + dav1d sources
- docs/issues/003-mixed-kernel-m4-bench.md: closure + matrix
- docs/k3_mc_phase7.md: M4 methodology caveat extended with V3/V4
- docs/k5_cdef_phase3_partial.md: deployment recommendation updated

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

docs/k3_mc_phase7.md

@@ -122,6 +122,27 @@ NEON-3 on kernel-A + QPU on kernel-B concurrently would close the
 question. ~½ day of additional bench work; would update the
 deployment recipe for cycles 3 + 5 if the result is positive.
 
+### Issue 003 results (2026-05-18, closed)
+
+`bench_concurrent_mixed` matrix in `docs/issues/003-mixed-kernel-m4-bench.md`
+confirms the methodology critique:
+
+| QPU side | CPU MC agg | per-core MC | QPU contribution |
+|---|---|---|---|
+| MC (V3 control, same kernel) | 22.64 Mblock/s | 7.5 avg | 0.39 Mblock/s MC |
+| LPF4 real QPU (V4) | **27.87 Mblock/s** | **9.3 avg** | **12.74 Medge/s LPF4** |
+
+Switching QPU off MC (same kernel) onto LPF4 (a different
+bandwidth-bound kernel) gave CPU MC **+23 % per-core uplift**.
+V4 = the actual daedalus-fourier deployment shape (CPU MC + QPU
+LPF4), and both substrates were productive concurrently.
+
+**Cycle 3 MC verdict unchanged**: QPU MC contributes ~0.4
+Mblock/s under any contention scenario (V3, V5). The 4 NEON cores
+do MC dramatically better. **MC stays on CPU.** But the
+*deployment recipe overall* (cycle 1+2+4 on QPU, 3 on CPU) is
+validated by V4 as a positive-sum arrangement.
+
 ## Decision per Phase 1 rules + 30fps-floor calibration
 
 | Rule | Result | Status |