…omes (#99 follow-on)

Closes the last remaining #99 follow-on: end-to-end integration coverage
for the three reconciliation outcomes the state machine can produce.

## Three scenarios added to engine::parallel::tests

Each exercises publish_package() against a real tiny_http mock registry
with a fake cargo binary configured to exit ambiguously (exit 1, empty
stdout/stderr — which classify_publish_failure maps to
CargoFailureClass::Ambiguous per existing unit tests).

Readiness polling is disabled (readiness.enabled = false) so each
reconcile site reduces to a single registry query — avoids flaky request
counts that a full polling loop would produce.

### 1. reconcile_bdd_ambiguous_resolves_to_published

- cargo fails ambiguously
- entry + post-cargo quick checks return 404
- reconcile's single version_exists poll returns 200
- expected: PackageState::Published, attempts=1 (no second cargo)
- verifies PublishReconciling + PublishReconciled{Published} events
  emitted to the on-disk event log

### 2. reconcile_bdd_ambiguous_resolves_to_not_published_then_retries

- cargo fails ambiguously on every attempt
- registry consistently returns 404
- reconcile resolves to NotPublished, falls through to normal retry
- after max_attempts=2, package ends Failed
- verifies PublishReconciled{NotPublished} event emitted
- verifies cargo was actually retried (attempts=2)

### 3. reconcile_bdd_resume_from_ambiguous_state_skips_republish

- state pre-seeded with PackageState::Ambiguous (simulating a prior
  interrupted run)
- entry check returns 404 (bypasses "already published" short-circuit)
- resume-path reconcile (from PR #115) fires BEFORE retry loop
- single version_exists query returns 200 → Published
- expected: attempts=0 (cargo NOT invoked)
- verifies via SHIPPER_CARGO_ARGS_LOG that cargo binary was never called
- verifies PublishReconciling + PublishReconciled{Published} events emitted

## Verification

- cargo test -p shipper --lib -- reconcile_bdd: 3 pass
- cargo test -p shipper --lib -- engine::parallel: 70 pass (no regressions)
- cargo clippy --all-targets -- -D warnings: clean
- cargo fmt --check: clean

## Why events are read from disk

publish_package flushes event batches to events.jsonl and clears the
in-memory log after each write. Assertions read via
EventLog::read_from_file(&events_path) to see the full persisted stream.

## #99 tracker after this merge

- [x] In-flight reconcile state machine (#111)
- [x] Resume-path reconciliation (#115)
- [x] Docs/contracts: cargo text is a hint, registry is truth (#117)
- [x] BDD integration scenarios for all 3 outcomes (this PR)

#99 is now fully closed.