Phase 8: wire IDCT QPU dispatch through public API

daedalus_ctx now owns a v3d_runner when V3D is available. The
public API's dispatch_vp9_idct8 routes QPU calls through a
new dispatch_idct8_qpu helper that: (1) lazy-creates the cycle 1
v4 pipeline on first use, (2) allocates 3 host-visible SSBOs
per call (coeffs/dst/meta), (3) memcpy host->GPU, (4) dispatch
with the v4 32-blocks-per-WG geometry, (5) memcpy GPU->host.

Per-call alloc is intentional for Phase 8 correctness-first
scope; buffer-pool perf optimization is deferred.

Added daedalus_ctx_create_no_qpu() for fast-path callers that
know they want CPU only.

test_api_idct extended to a 3-mode matrix: CPU forced, QPU
forced, AUTO recipe. All three deliver 4096/4096 bit-exact
on hertz with V3D 7.1.7.0:

  recipe substrate for VP9_IDCT8: 2 (QPU)
  [CPU] 4096/4096 bit-exact
  [QPU] 4096/4096 bit-exact (real QPU dispatch through the API)
  [AUTO] 4096/4096 bit-exact (recipe routes to QPU)

Next Phase 8 sub-step: same wiring pattern for cycle 2 LPF wd=4
and cycle 4 LPF wd=8 (the other two recipe-QPU kernels).
Cycle 3 MC and cycle 5 CDEF only need the dispatch hook
(recipe routes to CPU; QPU stays opportunistic via explicit
override).

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

include/daedalus.h

@@ -70,6 +70,10 @@ typedef struct daedalus_ctx daedalus_ctx;
  * failure. */
 daedalus_ctx *daedalus_ctx_create(void);
 
+/* Same but skip V3D init — for callers that know they want CPU
+ * only and want a fast-creating context. */
+daedalus_ctx *daedalus_ctx_create_no_qpu(void);
+
 /* Returns 1 if QPU dispatch is available on this context, 0 if
  * NEON-only.  Useful for the integration layer to short-circuit
  * QPU dispatch attempts. */