distillation: audit-baselines lineage port — fully closes the OPEN #2 surface

The original OPEN #2 line called for "SFT export pipeline +
audit_baselines lineage." Commit 7bb432f shipped the SFT export.
This commit ports the audit_baselines half — the longitudinal
drift signal that distinguishes "metrics shifted because the world
changed" from "metrics shifted because we broke something."

Mirrors Rust scripts/distillation/audit_full.ts's substrate:

- LoadLastBaseline(path) reads the most recent entry from
  data/_kb/audit_baselines.jsonl. Returns (nil, nil) on missing
  file (first run), errors on truncated last line (partial-write
  detection — operators don't lose drift signal silently).
- AppendBaseline(path, baseline) appends one entry as a JSON line.
  Atomic at the line level via bufio + O_APPEND. Creates the
  parent directory if missing.
- BuildAuditDriftTable(prior, current, threshold) computes
  per-metric drift. flag values mirror Rust exactly: first_run,
  ok, warn. DefaultDriftWarnThreshold = 0.20 = Rust's 20%.
- FormatAuditDriftTable renders a fixed-width text grid for
  stdout dumps in audit-full runs.

Edge cases handled:
- Zero-baseline: prior=0 means no division — PctChange stays nil.
  current=0 → ok (no change). current>0 → warn (zero→nonzero is
  always notable, never silently fine).
- New metric in current: flagged first_run, not "0%-change".
  Operators see "this is a new signal we haven't tracked before."
- Sort: stable by metric name for deterministic JSON output and
  clean CI diffs.

Generic on metric name (vs Rust's pinned p2_evidence_rows etc.):
the Rust phase numbering doesn't translate to Go directly. The
AuditBaselineRustCompat constant pins the Rust names so operators
running both runtimes use the same labels, which makes drift
comparison meaningful across the two pipelines.

13 new tests covering: missing file, last-line-wins, blank-line
tolerance, malformed-line errors, append round-trip, append-to-
existing, schema validation, first-run, threshold boundary,
zero-baseline, new-metric-in-current, sort-by-metric stability,
formatter output rendering.

OPEN #2's "audit_baselines lineage" half now closed. The
distillation package surface is at parity with the Rust pipeline:
scorer, scored runs, SFT export, audit baselines all available
on the Go side.

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

STATE_OF_PLAY.md

@@ -268,6 +268,7 @@ a steady state. Future items will land here as production triggers fire.
 | (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-2 lineage) | **Audit-baselines lineage ported** (2026-05-01): `internal/distillation/audit_baseline.go` mirrors Rust `audit_full.ts`'s LoadBaseline/AppendBaseline/buildDriftTable. `LoadLastBaseline` reads the most recent JSON line from `data/_kb/audit_baselines.jsonl`; `AppendBaseline` appends append-only with bufio. `BuildAuditDriftTable` flags drift `>20%` (configurable); zero-baseline and new-metric edge cases handled (no division-by-zero, no false-stable on zero→nonzero). `FormatAuditDriftTable` for stdout dumps. Generic on metric names so callers running both runtimes can pin Rust-compat names (`AuditBaselineRustCompat` constant lists them). 13 tests including last-line-wins, trailing-blank-tolerance, malformed-line-errors, threshold-boundary, zero-baseline-handling, sort-stability. |
 | (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. |