ds4_test fails 29 fusion assertions on M4 — fusion whitelists only admit M3/M5 device names

On an M4 Max (128 GiB), `make test` with the DS4F 0731 IQ2XXS GGUF fails 29 assertions in ds4_test, in two clusters: `ds4_gpu_matmul_f16_pair_compressor_store_tensor` (ds4_test.c:1098) and the forced `ds4_gpu_flash_kv_stage_f16_tensor` path (ds4_test.c:2728), plus the mismatch-count assertions that follow once the fused output is never written.

Root cause: the newest decode ports gate on `ds4_gpu_ported_m5_decode_feature_enabled()`, whose pre-M5 predicate admits M1–M4, but the older fusion sites still use literal `ds4_gpu_device_name_contains("M3") || ("M5")` whitelists. On M4 those entry points return 0 by design ("not applicable"), and the tests assert they succeed without replicating the device gate. M1/M2 should fail the same way.

It's not only a test problem: the fifteen name-gated fusions never engage on M4, and several are real wins there (numbers in #770).

#770 admits M4 to the fifteen sites; with it the full suite is green on M4 Max and every fused-vs-reference comparison is bit-exact.