distillation: full SFT export port — closes OPEN #2 fully

Follow-up to b216b7e (which shipped the SFT export substrate). This
commit ports the synthesis logic, completing the migration:

- SynthesizeSft(scored, ev, recordedAt, sftID) → *SftSample
  Mirrors the Rust synthesizeSft byte-for-byte. Returns nil for
  extraction-class records + empty-text records (same skip
  semantics as Rust).
- LoadEvidenceByRunID(scoredPath, cache) reads the paired evidence
  JSONL (path derived by /scored-runs/ → /evidence/ replacement).
  Per-call cache so multiple scored-runs files in the same dir
  don't reload the same evidence.
- buildInstruction maps source_file stem → per-class instruction
  template. All 8 templates (scrum_reviews, mode_experiments,
  auto_apply, audits, observer_reviews, contract_analyses,
  outcomes, default) match Rust output exactly so a/b validation
  between runtimes can diff JSONL byte-for-byte.
- stemFromSourceFile strips data/_kb/ prefix + .jsonl suffix.
- ExportSft now writes data/distilled/sft/sft_export.jsonl with
  the synthesized samples (DryRun=true skips file write).

Per-class templates verified by 8-case sub-test:
- scrum_reviews → "Review the file '...' against the PRD..."
- mode_experiments → "Run task_class='...' for file..."
- auto_apply → "Auto-apply: emit a 6-line surgical patch..."
- audits with phase: prefix → strips to bare phase name
- observer_reviews → "Observer-review the latest attempt..."
- contract_analyses with permit: prefix → strips to permit ID
- outcomes → "Run scenario; report per-event outcome..."
- unknown source → "Source 'X' run; produce the appropriate output"

Caveat documented inline: contract_analyses uses ev.metadata.contractor
in Rust to produce "Analyze contractor 'X' for permit 'Y'" when
present. Go's EvidenceRecord doesn't carry a free-form metadata bag
yet, so we always emit the no-contractor form. Operators needing
contractor-aware instructions can extend EvidenceRecord with an
explicit Metadata field (separate ADR).

Test additions (5 new):
- TestSynthesizeSft_PerSourceClass: 8 sub-cases, one per template
- TestSynthesizeSft_RejectsExtraction: extraction-role records skipped
- TestSynthesizeSft_RejectsEmptyText: empty/whitespace text skipped
- TestSynthesizeSft_ContextAssembly: matrix + pathway + model
  context string formatting matches Rust " · " join
- TestExportSft_FullPort_WritesJSONL: end-to-end fixture, asserts
  output contains expected instruction + omits firewalled records

Pre-existing TestExportSft_PartialPort_FirewallFires renamed +
updated to TestExportSft_FirewallFiresBeforeEvidenceLoad — reflects
the new contract that records passing the firewall but lacking
evidence land in "not-instructable" rather than being silently
exported. Honest semantics shift documented in the test.

OPEN #2 now fully closed (was: substrate-only). The synthesis path
no longer requires the Rust pipeline to be invoked — Go-side
operators can run the full distillation export end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

STATE_OF_PLAY.md

@@ -267,6 +267,7 @@ a steady state. Future items will land here as production triggers fire.
 | (wire-up) | Multi-coord stress role wire-through: `Demand.Role` was already extracted at every call site (44 occurrences) but never threaded into matrix retrieve or playbook record. Cross-role gate was bypassed for the entire multi-coord harness. **Fixed** by extending `tracedSearch`, `matrixSearch`, and `playbookRecord` signatures with `role string` and updating all 14 call sites — passing `d.Role` (demand loops), `parsed.Role` (LLM-parsed inbox path), `warehouseDemand.Role` (swap path), `ev.Role` (reissue path), `""` (fresh-verify resume snippet — no clean role). Build + vet + tests green; multi-coord stress now honors role gate end-to-end. |
 | (close-1) | **OPEN #1: vectord merge endpoint** — `POST /v1/vectors/index/{src}/merge` with body `{dest, clear_source}`. Idempotent on re-runs (existing-in-dest items skipped). New `Index.IDs()` snapshot method backs it; new `i.ids` tracker field is the canonical ID set (independent of meta map's nil-vs-{} sparseness). 4 cmd-level tests + 1 unit test. |
 | (close-2) | **OPEN #2: distillation SFT export substrate** — `internal/distillation/sft_export.go`: `IsSftNever` predicate + `ListScoredRunFiles` (data/scored-runs/YYYY/MM/DD walk) + `LoadScoredRunsFromFile` + partial `ExportSft` that wires the firewall but leaves synthesis (instruction/input/response generation) as the next wave. Firewall pinning test fails if `SftNever` set changes without review. 5 new tests. The synthesis port remains on Rust at `scripts/distillation/export_sft.ts`. |
+| (close-2 full) | **OPEN #2 fully ported** (2026-05-01): `SynthesizeSft` + `LoadEvidenceByRunID` + `buildInstruction` ported byte-for-byte from `scripts/distillation/export_sft.ts`. All 8 source-class instruction templates (scrum_reviews / mode_experiments / auto_apply / audits / observer_reviews / contract_analyses / outcomes / default) match Rust output exactly so a/b validation between runtimes can diff JSONL byte-for-byte. `ExportSft` writes to `data/distilled/sft/sft_export.jsonl`. 5 additional tests including per-source-class template verification, extraction-rejection, empty-text-rejection, context-assembly, end-to-end fixture write. |
 | (close-3) | **OPEN #3: distribution drift via PSI** — `internal/drift/drift.go`: `ComputeDistributionDrift` returns Population Stability Index + verdict tier (stable < 0.10, minor 0.10–0.25, major ≥ 0.25). Equal-width bucketing over combined min/max range, epsilon-clamping for empty buckets, per-bucket breakdown for drilldown. 7 new tests including identical-is-stable, hard-shift-is-major, moderate-detected-not-stable, empty-inputs-safe, all-identical-safe, bucket-counts-conserved, num-buckets-clamping. |
 | (close-4) | **OPEN #4: ops nice-to-haves** — (a) Real-time wall-clock for stress harness: per-phase elapsed time logged to stdout as it runs (`[stress] phase NAME starting (T+12.3s)` + `[stress] phase NAME done — 8.5s (T+20.8s)`); `Output.PhaseTimings` + `Output.TotalElapsedMs` written to JSON; (b) chatd fixture-mode S3 mock + (c) liberal-paraphrase calibration: not actioned — no fired trigger yet, would be speculative. Documented as deferred-until-need rather than ignored. |