…-MOE-SUB-1

Per `contracts/trace-moe-gpu-sub-stages-v1.yaml` v1.0.0 (PR #1498),
M-MOE-SUB-1 implementation: extend the parent SaveTensorStage enum
with the 2 new mandatory MoE-GPU bisection variants.

WHAT THIS PR ADDS:

  * `MoeRouter` — top-k expert weights post-softmax + renormalize,
    per-layer, captured AFTER FfnNorm and BEFORE per-expert SwiGLU.
  * `MoeFfnOut` — aggregated MoE FFN output Σ w_e * expert_out_e,
    per-layer, captured AFTER all per-expert computations.

CHANGES:

  * `SaveTensorStage` enum: 2 new variants between `PostFfnResidual`
    and `FinalNorm`.
  * `ALL` array: 20 → 22 entries; new variants in canonical position.
  * `canonical_name()`: 2 new arms returning "moe_router" + "moe_ffn_out".
  * `FromStr::from_str()` lowercase match: 2 new accepted strings.
  * `is_per_layer_count_matches_contract` test: per_layer 18 → 20,
    total 20 → 22.
  * `canonical_names_match_contract_enumeration` test: expected[]
    grows to 22 with new entries in canonical order.
  * 5 new `falsify_moe_sub_001_*` tests:
      - `moe_router_round_trip`
      - `moe_ffn_out_round_trip`
      - `2_new_stages_in_canonical_order` (ALL position assertions)
      - `parse_list_accepts_2_new_stages_together`
      - `parse_list_accepts_full_moe_block_chain` (3-stage bisection
        chain `ffn_norm,moe_router,moe_ffn_out`)

DISCHARGES: FALSIFY-MOE-SUB-001 (per contract).

NOT DISCHARGED YET (separate follow-up scope per contract):

  * FALSIFY-MOE-SUB-002 (byte-identity preservation) — needs the
    instrumentation PR (M-MOE-SUB-2 + 3) that wires MoeRouter +
    MoeFfnOut capture into both forward bodies.
  * FALSIFY-MOE-SUB-003 (live bisection on lambda-vector RTX 4090).
  * FALSIFY-MOE-SUB-004 (fix PR cites bisected stage by name).

VERIFICATION:

  cargo test -p aprender-serve --lib save_tensor_stage
  → 29 passed; 0 failed; 0 ignored

  rustfmt --check  → exit 0

DEPENDS ON: PR #1498 (`trace-moe-gpu-sub-stages-v1` v1.0.0 contract
scaffold) being on aprender main first. The contract pinned the
new variants' names + semantics; this PR implements them.

Refs: M-GPU-MOE-1.4 step (a) instrumentation, R10,
      qwen3-moe-forward-gpu-v1 v1.4.0,
      trace-moe-gpu-sub-stages-v1 v1.0.0,
      FALSIFY-MOE-SUB-001 (DISCHARGED).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>