Adds gemm_cache_bias field, create_cached_fwd_gemm_desc_bias builder,
and sgemm_f32_fused_bias helper. Pre-creates descriptors for every
BIAS-only output shape used in the forward pass. No call sites
migrated yet — subsequent commits flip individual sites to use the
fused helper.
Mirrors the established RELU_BIAS infrastructure pattern: deterministic
algorithm selection, per-call bias-pointer wiring, Err-on-missing-cache
contract for graceful fall-back.
50e6cdc906