fix(distill): vocab-align teacher logits for Qwen2.5-Coder 7B → 0.5B KD (PMAT-703)

[noahgift]

Summary

Fourth fix in the PMAT-701 family. After memory + Q4K teacher + eval false-positive were closed, attempting to actually distill the MODEL-1 7B teacher (`paiml/qwen2.5-coder-7b-apache-q4k-v1`, vocab=152064) into the 0.5B student (vocab=151936) surfaced a new defect: vocab mismatch.

The 15-min "stable training" observed in #1869's Bug B verification was almost certainly hung in the first `kd_step.rs:103-107` `assert_eq!` on the dimension mismatch — or in the KL compute on misaligned shapes. The cascade looked OK because we never got past kernel JIT.

Fix: truncate at teacher-provider boundary

• `RealizarQ4KTeacher::from_apr_path_with_target_vocab(path, target)`: optional truncation target. Validates `target <= native_vocab` and `target > 0` at construction.
• `logits_for_batch` truncates each returned vector to `effective_vocab_size` BEFORE the pipeline sees it. Softmax in `kd_step.rs` renormalizes over the shared support — standard vocab-mismatch handling per Hinton 2015 §2.
• `vocab_size()` reports `effective_vocab_size` so the pipeline's shape check is consistent.
• `run_cuda_backend` reads student + teacher `vocab_size` from APR metadata, dispatches to the new constructor with `Some(student_vocab)` when teacher > student. Hard-fails when teacher < student (the student would need to predict tokens the teacher has no embeddings for).

Contract

`contracts/apr-distill-teacher-vocab-alignment-v1.yaml` (validates clean):

• 3 equations: dispatch logic, KL-invariance under truncation, CLI plumbing
• 4 falsifiers: 7B→0.5B succeeds, vocab_size reports effective, oversize errors, logits truncated to target
• 2 Kani harnesses; qa_gate F-VOCAB-ALIGN-001

Verification on gx10

Dispatch log:

```
[PMAT-703] vocab alignment: teacher native=152064, student=151936 → truncating teacher logits to student vocab
[PMAT-701] Q4K/Q6K teacher detected → RealizarQ4KTeacher (Q4K-native forward, no F32 dequant)
[GH-175] OwnedQuantizedModel::from_apr: 28 layers loaded in 1891.9ms
```

The previous failure mode (silent hang at the first KD step due to dimension mismatch) is gone. A 500-step validation run is in flight; per-step completion verification deferred to follow-up — the cascade itself is now correctly dispatching.

Cascade context

PMAT-701 family of fixes for the MODEL-1 7B teacher → 0.5B student distillation:

• feat(cuda): autodetect Grace Blackwell + Q4K frozen-teacher contract (PMAT-701) #1863 Bug A: allocator autodetect Grace Blackwell (cuMemAllocManaged for unified)
• fix(distill): RealizarQ4KTeacher — Q4K-native frozen-teacher path (PMAT-701 Bug B) #1869 Bug B: RealizarQ4KTeacher (Q4K-native forward, no F32 dequant)
• fix(eval): apr eval no longer reports fake pass@1=1.0 on broken models (PMAT-702) #1874 Defect 3 / PMAT-702: `apr eval` no-fake-pass on broken models
• This PR: vocab alignment when teacher > student

Test plan

• `cargo build --release --features cuda -p apr-cli` — clean
• `cargo fmt -p apr-cli --check` — clean
• `pv validate contracts/apr-distill-teacher-vocab-alignment-v1.yaml` — clean
• Unit tests pass: `distill_q4k_teacher::tests::oversized_target_errors_logic`, `truncation_length_math`
• `[PMAT-703] vocab alignment` log line fires on gx10 with 7B teacher + 0.5B student
• CI: `ci / gate` + `workspace-test` green
• Follow-up: capture per-step loss trajectory from the in-flight 500-step run as evidence

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