h264: qpel avg anchors (avg_mc20/02/22, biprediction support)

Begins the avg_ qpel buildout for B-slice biprediction.  Each avg_
form computes the same half-pel formula as its put_ sibling, then
L2-averages the result with the existing dst contents — the caller
pre-loads dst with the list0 prediction; the avg_ call adds list1
per H.264 §8.4.2.3.1.

Scope (3 anchors, sets the pattern for the remaining 13 avg_
variants):
  - 3 new kernel enums (AVG_MC20=31, AVG_MC02=32, AVG_MC22=33) → CPU.
  - 3 NEON externs for the vendored ff_avg_h264_qpel8_{mc20,mc02,mc22}_neon.
  - 3 CPU dispatches via existing DEFINE_QPEL_CPU_DISPATCH macro
    (the macro is type-agnostic so it didn't need changes for avg_).
  - 3 public dispatches via DEFINE_QPEL_DISPATCH macro.
  - 3 recipe wrappers via DEFINE_QPEL_RECIPE macro.
  - tests/h264_qpel8_avg_anchors_ref.c — per-cell helpers + L2 avg.
  - Test harness: run_avg_qpel() seeds dst with random content so
    the L2 averaging is actually exercised (not just put_-style
    overwrite that would silently pass).

Verified on hertz:

  $ ./build/test_api_h264 | tail -3
    H.264 qpel avg_mc20: 2048/2048 bytes bit-exact (100.0000%)
    H.264 qpel avg_mc02: 2048/2048 bytes bit-exact (100.0000%)
    H.264 qpel avg_mc22: 2048/2048 bytes bit-exact (100.0000%)

  All 3 anchors bit-exact PASS first try.

Why anchors only in this PR: the avg_ pattern is uniform across all
16 positions (each is just "put_ result + L2 with dst").  Landing
the anchors first confirms the macro pattern works for both put_
and avg_; the remaining 13 (avg_mc10/30/01/03 + avg_mc11..33) follow
the same template in a follow-up PR.

State of the qpel matrix after this PR:
  put_ : 15 of 16 positions ✓ (mc00 is integer copy, no wrapper)
  avg_ :  3 of 16 positions ✓ (mc20, mc02, mc22 anchors)
        13 follow-up positions

CMakeLists.txt

@@ -527,6 +527,7 @@ add_executable(test_api_h264
     tests/h264_qpel8_mc22_ref.c
     tests/h264_qpel8_quarter_axis_ref.c
     tests/h264_qpel8_diag_ref.c
+    tests/h264_qpel8_avg_anchors_ref.c
 )
 target_link_libraries(test_api_h264 PRIVATE daedalus_core)
 target_compile_options(test_api_h264 PRIVATE -O2)