…1481)

Adds the read-half of `apr pretrain --init` weight load: a thin
wrapper over `aprender::format::converter::load_model_tensors` that
returns a `BTreeMap<String, (Vec<f32>, Vec<usize>)>` of tensor blobs
keyed by HF naming convention.

Per `apr-pretrain-arch-polymorphic-v1` §init_load_semantics (PR #1473):
"Loader is REUSED, not reimplemented." This function does not duplicate
APR parsing — it forwards to the same machinery `apr export` and
`apr inspect` use.

Discharges from `apr-pretrain-arch-polymorphic-v1`:
  - §init_load_semantics invariant (loader reuse): satisfied
  - FALSIFY-006 (init_loss < 6.0) at READ-COMPILE-BIND level

Step 5f decomposition:
  - 5f.1 (PR #1479): encoder/decoder family validator (~30 LOC)
  - 5f.2 (this PR): APR file open + tensor read (~30 LOC + 2 tests)
  - 5f.3 (next):    populate trainer parameters from BTreeMap (~50 LOC)
  - 5g  (operator): LIVE 500-step fine-tune → DISCHARGES MODEL-2 ship-%

Step 5f.2 is intentionally narrow — it only does the READ. Population
into trainer parameter slots (5f.3) reconciles HF naming convention
(e.g., `model.embed_tokens.weight`) against the trainer's internal
parameter naming. That's a separate concern with its own falsifier.

What this PR adds:

  1. `pub fn load_init_tensors_from_apr(path) -> Result<BTreeMap<...>>`
     at pretrain_real.rs:35 (~25 LOC including doc comment)
  2. 2 unit tests in `pretrain_real::tests`:
       - load_init_tensors_missing_file_errors_with_falsifier_id
         (FALSIFY-006 fail-fast path; asserts error message contains
          falsifier id + offending path for operator-experience)
       - load_init_tensors_signature_compile_bind
         (drift-prevention: catches a future signature change that
          would break step 5f.3's BTreeMap consumer)

Test results (cargo test -p aprender-train --lib train::pretrain_real::tests::load_init_tensors):
    2 passed; 0 failed; 0 ignored

Five Whys:

  1. Why decompose step 5f.2 to JUST the read? Single-piece flow.
     Read → Validate → Populate are three distinct concerns. Step 5f.1
     did validation (#1479); 5f.2 does read; 5f.3 will do populate.
     Each PR has one falsifier discharge story.

  2. Why use load_model_tensors and not write a new parser? The contract
     pins "Loader is reused, not reimplemented." Writing a new parser
     would create a parallel format-decoder that drifts from the canonical
     one. The same lesson as the LAYOUT-001/002 hits — parallel format
     code paths produce silent format-drift bugs.

  3. Why return BTreeMap<String, (Vec<f32>, Vec<usize>)> rather than a
     trainer-parameter-shaped struct? Decoupling: the read shouldn't
     know about TransformerTrainer's internal parameter names. Step
     5f.3's job is to map HF names → trainer slots; if 5f.2 baked that
     mapping in, every change to TransformerTrainer would break the read.

  4. Why include the signature-compile-bind test? It's a compile-time
     check that drives step 5f.3's expectations. If a future refactor
     changes the return type (e.g., from BTreeMap to HashMap, or from
     Vec<usize> to Box<[usize]>), step 5f.3's consumer code stops
     compiling — caught here, not at the integration point.

  5. Why is FALSIFY-006 NOT yet at PARTIAL_ALGORITHM_LEVEL after this
     PR? Because step 5f.2 only does the read; FALSIFY-006 requires
     the LIVE init_loss < 6.0 check, which needs steps 5f.3 + 5g.
     This PR moves FALSIFY-006 from UNBOUND → READ-COMPILE-BIND, a
     sub-level of PARTIAL_ALGORITHM_LEVEL. Full PARTIAL discharge
     happens at 5f.3 when the populate step exists.

Plain ship-% update:
  - MODEL-1: unchanged at 91% (SHIP-007 cascade infrastructure track)
  - MODEL-2: unchanged at 57% — first ship-% movement gated on §50.4
    step 5g (LIVE 500-step fine-tune producing val_loss < 9.38)

Refs:
  - SPEC-SHIP-TWO-001 §50, §51 — MODEL-2 architecture-coupling +
    cascade snapshot (PR #1472, #1480 MERGED)
  - PR #1473 — apr-pretrain-arch-polymorphic-v1 contract (in flight)
  - PR #1474 — qwen2_0_5b tie_word_embeddings fix (MERGED)
  - PR #1475 — build_transformer_config polymorphic dispatch (in flight)
  - PR #1476 — preflight_tokenizer_vocab_matches_target (MERGED)
  - PR #1478 — GQA-7:1 forward-pass smoke test (MERGED)
  - PR #1479 — validate_pretrain_init_arch_compatible (in flight)
  - feedback_no_guessing.md
  - feedback_falsifier_first_cascade_pattern.md (this turn's pattern)

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>