Replaces CheckpointV1 with CheckpointV2 — covers the full v2 inference
graph: VSN, Mamba2 stacks 1+2 (in/a/b/c/out weights + biases), LN_a/LN_b,
attention-pool, CfC (now v2-shaped via cfc_n_in=HIDDEN_DIM), and the
full GRN heads (10 tensors: w1/b1, w2/b2, w_gate/b_gate, w_main/b_main,
w_skip/b_skip).
save_checkpoint reads every trunk weight tensor via memcpy_dtoh and
packs into the CheckpointV2 bincode envelope. load_checkpoint peeks
the version first (CheckpointVersionProbe), rejects non-2 versions,
deserialises into CheckpointV2, validates n_in / n_hid / cfc_n_in /
mamba2_state_dim match the supplied cfg, and uploads each weight
tensor with size-checked memcpy_htod.
V1 envelopes hard-rejected — alpha_train never produced V1 files, so
no migration. The old V1-shaped roundtrip test is removed; new V2
round-trip test will land alongside the alpha_train wiring (X14).
Verification:
- perception_forward_golden: PASS (max_diff = 0.000000)
- ml-alpha lib tests: 34 pass
- fxt-backtest binary builds clean
Per spec §1.2 (X12).
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