feat(distill): wire ProgressCallback into Pipeline — close training-monitoring gap (PMAT-705)

[noahgift]

Summary

Closes the training-monitoring gap surfaced by the PMAT-704 cascade. A 1.5 h gx10 hang at step 0 went unnoticed because the distill pipeline emits zero output between "blocks uploaded" and "Distillation complete."

Root cause: `aprender-train` ships a full callback infrastructure (`ProgressCallback`, `MonitorCallback`, `CheckpointCallback`, etc.) but `aprender-train-distill::Pipeline` had zero callback references. Per-step loss was computed in `kd_step` and discarded.

Fix

`crates/aprender-train-distill/src/pipeline.rs`:

• `Pipeline` gains `callbacks: Vec<Box>` field + `with_callback()` builder.
• Training loop wires full lifecycle: `on_train_begin` → `on_epoch_begin` → `on_step_end` × N (with current loss + step counter) → `on_epoch_end` → `on_train_end`.
• `CallbackAction::Stop` terminates the loop within one step (early-stopping support).

`crates/apr-cli/src/commands/distill.rs::run_cuda_backend`:

• Attaches default `ProgressCallback` honoring `APR_DISTILL_LOG_EVERY` env (default 10). Set to 0 to disable per-step logs (epoch boundaries still log).

Contract

`contracts/distill-pipeline-observability-v1.yaml` (validates clean):

• 3 equations: callback lifecycle, progress log format, default attachment
• 4 falsifiers: per-step callback fires; log_every honored; APR_DISTILL_LOG_EVERY=0 silences; overhead bounded
• 2 Kani harnesses (callback ordering, Stop terminates loop)
• qa_gate F-OBSERV-001

Unit tests

3 new tests in `pipeline::tests::pmat_705_observability`, all PASS:

```
falsify_observ_001_per_step_callback_fires .... ok
falsify_observ_001_step_count_matches ......... ok
callback_ordering_preserved ................... ok
```

Use a recording callback that captures every lifecycle event and verify counts match (epochs × steps_per_epoch).

gx10 verification

Dispatch emits:

```
[PMAT-705] ProgressCallback attached (log every 1 steps)
```

Full per-step output on the 7B teacher path needs #1879 (PMAT-704) to land too — current main routes Q4K through Bug B's CPU-bound path which hangs at step 0 (no steps → no logs). The PMAT-705 wiring is correct independent of teacher backend; unit tests prove it.

Cascade

Sixth fix in the PMAT-701 family:

• feat(cuda): autodetect Grace Blackwell + Q4K frozen-teacher contract (PMAT-701) #1863 Bug A: allocator autodetect
• fix(distill): RealizarQ4KTeacher — Q4K-native frozen-teacher path (PMAT-701 Bug B) #1869 Bug B: RealizarQ4KTeacher (demoted by fix(distill): default Q4K teacher to CudaTrainerTeacher (cuBLAS) — revert Bug B's slow path (PMAT-704) #1879)
• chore(distill): default to MODEL-1 7B teacher + SPEC-DISTILL-001 §86 (PMAT-701 follow-up) #1871 SPEC-DISTILL §86
• fix(eval): apr eval no longer reports fake pass@1=1.0 on broken models (PMAT-702) #1874 PMAT-702: apr eval no-fake-pass
• fix(distill): vocab-align teacher logits for Qwen2.5-Coder 7B → 0.5B KD (PMAT-703) #1877 PMAT-703: vocab alignment (superseded by fix(distill): default Q4K teacher to CudaTrainerTeacher (cuBLAS) — revert Bug B's slow path (PMAT-704) #1879)
• fix(distill): default Q4K teacher to CudaTrainerTeacher (cuBLAS) — revert Bug B's slow path (PMAT-704) #1879 PMAT-704: cuBLAS default
• docs(spec): SPEC-DISTILL-001 §87 — PMAT-704 post-mortem on Bug B wrong turn #1880 SPEC-DISTILL §87 post-mortem
• This PR: PMAT-705 ProgressCallback

Together the distill → eval pipeline is honest AND observable end-to-end.

Test plan

• `cargo build --release --features cuda -p apr-cli` — clean
• `cargo test -p aprender-train-distill --lib pipeline::tests::pmat_705` — 3/3 PASS
• `cargo fmt -p aprender-train-distill -p apr-cli --check` — clean
• `pv validate contracts/distill-pipeline-observability-v1.yaml` — clean
• gx10 dispatch confirms `[PMAT-705] ProgressCallback attached` marker fires
• CI: `ci / gate` + `workspace-test` green
• Follow-up: capture per-step loss trajectory from a 7B run once fix(distill): default Q4K teacher to CudaTrainerTeacher (cuBLAS) — revert Bug B's slow path (PMAT-704) #1879 lands

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