Adds the 4 missing GRN head fields (heads_w1, heads_b1, heads_w2, heads_b2)
that X1's skeleton under-modeled. Trunk now owns all 10 GRN head tensors:
input projection (HIDDEN→HEAD_MID), mid→mid layer, gate/main/skip outputs.
Trainer's 10 head fields removed. Trainer init still draws heads from
its ChaCha8Rng chain at the same call position, then memcpy_htod's them
into the trunk's now-allocated slots. Forward, backward, and AdamW
access sites redirect to self.trunk.heads_*. Gradient buffers + AdamW
state stay at trainer level.
Verification:
- perception_forward_golden: PASS (max_diff = 0.000000)
- ml-alpha lib tests: 34 pass
Per spec §2.2 (X9). After this commit, the trunk owns ALL v2 inference
weights — the source of truth for downstream checkpoint serialization.
5bd8897880