Adds zero-initialised v2 weight tensors (VSN, LN_a/b, attn_q, GRN heads)
and Option<Mamba2Block> slots for stacks 1 and 2 to CfcTrunk. Extends
CfcConfig with mamba2_state_dim (default 16, matches
PerceptionTrainerConfig::default). Imports HEAD_MID_DIM for GRN head
sizing.
No forward path changes — fields allocated, not yet read. Existing
new_random init for V1 fields (CfC + simple heads + projection) is
preserved unchanged so the trunk's forward kernels still produce the
same output.
Skeleton for X2..X9 weight-group migrations.
Verification:
- ml-alpha lib tests: 34 pass
- perception_forward_golden bit-equivalence: PASS (max_diff = 0.000000)
Per spec 2026-05-19-ml-alpha-v2-trunk-grows-and-deployability-design.md
§1.1, §2.2 (X1).
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