validatord: coordinator session JSONL for offline analysis (B follow-up)

Closes the second half of J's 2026-05-02 multi-call observability
concern. Trace-id propagation (commit d6d2fdf) gave us the *live*
view in Langfuse; this gives us the *longitudinal* view for ad-hoc
DuckDB queries over thousands of sessions:

  "show me every session where the model produced a real candidate
   without ever needing a retry"
  "find sessions where validation rejected three times in a row"
  "first-shot success rate per model — did we feed it enough corpus?"

## What's in

internal/validator/session_log.go:
  - SessionRecord type (schema=session.iterate.v1)
  - SessionLogger writer — mutex-guarded append, best-effort posture,
    nil-safe (NewSessionLogger("") = nil = no-op on Append)
  - BuildSessionRecord helper — assembles a row from any
    iterate response/failure/infra-error combination, callable from
    other daemons that wrap iterate (cross-daemon shared schema)
  - 7 unit tests including concurrent-append safety + the three
    code paths (success / max_iter_exhausted / infra_error)

cmd/validatord/main.go:
  - handlers.sessionLog field + wiring from cfg.Validatord.SessionLogPath
  - Iterate handler: build + append a SessionRecord on every call
  - rosterCheckFor("fill") closure stamps grounded_in_roster — the
    load-bearing forensic property J flagged ("we can never
    hallucinate available staff members to contracts")

internal/shared/config.go + lakehouse.toml:
  - [validatord].session_log_path field; empty = disabled
  - Production: /var/lib/lakehouse/validator/sessions.jsonl

scripts/validatord_smoke.sh:
  - Adds a probe verifying validatord announces session log path on
    startup. Smoke is now 6/6 (was 5/5).

docs/SESSION_LOG.md:
  - Schema reference + 5 worked DuckDB query examples including the
    "alarm" query (sessions where grounded_in_roster=false on an
    accepted fill — should always be empty; if not, something is
    bypassing FillValidator).

## What this is NOT

This is NOT a duplicate of replay_runs.jsonl. They're siblings:
  - replay_runs.jsonl: replay tool's per-task retrieval+model output
  - sessions.jsonl: validatord's per-iterate full retry chain +
    grounded-in-roster verdict

A single coordinator session can produce rows in both streams; the
session_id (= Langfuse trace_id) is the join key.

## Layered observability now in place

  Live view:  Langfuse trace tree (X-Lakehouse-Trace-Id propagation)
              `iterate.attempt[N]` spans with prompt/raw/verdict
  Offline:    coordinator_sessions.jsonl (this commit)
              DuckDB-queryable; longitudinal forensics
  Hard gate:  FillValidator + WorkerLookup (existing)
              phantom IDs structurally rejected, never reach
              session log's grounded_in_roster=true bucket

Per the architecture invariant in STATE_OF_PLAY's DO NOT RELITIGATE
section — these layers are wired; future work targets the data, not
the wiring.

## Verification

- internal/validator: 7 new tests (session_log_test.go) — all PASS
- cmd/validatord: 3 new integration tests covering the success,
  failure, and grounded=false paths — all PASS
- validatord_smoke.sh: 6/6 PASS through gateway :3110
- Full go test ./... green across 33 packages

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

internal/shared/config.go

@@ -165,6 +165,12 @@ type ValidatordConfig struct {
 	DefaultMaxTokens int `toml:"default_max_tokens"`
 	// Per-call timeout for the chat hop in seconds. 0 = 240s.
 	ChatTimeoutSecs int `toml:"chat_timeout_secs"`
+	// SessionLogPath: where to append SessionRecord JSONL rows for
+	// offline analysis (DuckDB queries, scrum review tooling). Empty
+	// = disabled. Production sets a stable path under
+	// /var/lib/lakehouse/validator/sessions.jsonl. Append-only,
+	// best-effort; see internal/validator/session_log.go.
+	SessionLogPath string `toml:"session_log_path"`
 }
 
 // ObserverdConfig drives the observer service (cmd/observerd).
@@ -393,6 +399,7 @@ func DefaultConfig() Config {
 			DefaultMaxIterations: 3,
 			DefaultMaxTokens:     4096,
 			ChatTimeoutSecs:      240,
+			SessionLogPath:       "", // empty = no session JSONL. Operators set under /var/lib/lakehouse/validator/sessions.jsonl.
 		},
 		Chatd: ChatdConfig{
 			Bind:               "127.0.0.1:3220",