… (PMAT-706)

#1888 (commit 52650c6) announced the PMAT-706 smoke-validation mode but the
squash landed ONLY contracts/apr-distill-smoke-validation-v1.yaml — the
pipeline.rs implementation described in the commit message never existed in
source (`git grep APR_DISTILL_MAX_STEPS` at HEAD hit only CHANGELOG, the
contract, and the resume doc; the commit's --stat was "1 file changed"). The
60s fail-fast gate the entire distillation critical path is supposed to run
before a 30-50h Stage D job was therefore absent.

Re-land against the existing contract (the behavioral source of truth):

- `Pipeline` gains a `smoke_max_steps: Option<u64>` field, read once from
  `APR_DISTILL_MAX_STEPS` at construction (so the CLI dispatch picks it up
  with no changes) and overridable via `with_max_steps()` — a test seam so
  the falsifiers never touch process env (this repo has repeatedly been bitten
  by env-var races in parallel tests).
- `train()`: early-breaks the inner loop after exactly N steps (step >= N, no
  off-by-one), rejects N=0 with a clear ConfigValue error, guards the
  zero-steps-completed load-failure case, and prints the two `[SMOKE]` summary
  lines ONLY on the early-break path.
- `execute()`: skips the export side-effect in smoke mode — no
  model.safetensors / output.apr so `apr eval`/`apr run` can't consume a
  smoke result by accident.
- `smoke_summary_lines()` factored out as a pure, total function so the exact
  wire format is unit-tested without capturing stdout.
- `scripts/dispatch-distill-stage-d.sh`: forward APR_DISTILL_MAX_STEPS across
  the ssh/`env` boundary, echo it, and add it to the JSON manifest +
  usage header — the documented `APR_DISTILL_MAX_STEPS=10 ./scripts/...` was
  previously a silent no-op over ssh.

Falsifiers (all passing, paths match the contract's `evidence:` exactly):
  pipeline::tests::pmat_706_smoke               (FT-SMOKE-001: exactly N steps)
  pipeline::tests::pmat_706_no_regression       (FT-SMOKE-002: unset = full run + export)
  pipeline::tests::pmat_706_summary_format      (FT-SMOKE-003: two parseable lines)
  pipeline::tests::pmat_706_no_output_in_smoke  (FT-SMOKE-004: no artifact written)
  pipeline::tests::pmat_706_zero_steps_is_clear_error  (KANI-SMOKE-002 analog)
  pipeline::tests::pmat_706_parse_max_steps_value      (env-var precondition)

70/70 distill lib tests pass, pipeline.rs clippy-clean, pv validate OK,
1392 aprender-contracts tests pass. Found via the repo-roadmap analysis
(critical-path first link). See feedback_squash_merge_post_verify.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>