Add the ledger: the audit writer and session state behind one lock

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

docs/implementer-log.md

@@ -6,6 +6,7 @@ reviewer adds findings under "Reviews" once per milestone.
 
 | Task | Date | Status | Gate runs | First gate | Deviations | Notes | Model |
 |---|---|---|---|---|---|---|---|
+| M3a/12-brokerd-ledger | 2026-09-20 | done | 2 | fail | none | Wrote crates/brokerd/src/ledger.rs (499 lines): Ledger + Inner { audit, state, stopped } behind one Mutex, and the three steps that hold it. decide copies the request out, reads state then policy::decide, and records the outcome (allowed/ask/denied, grant fields set together) as AuditEvent::Decision; answer re-decides an approval (approved only) and records AuditEvent::Approval with the answer/by/reason; finish raises the state for a Result and records AuditEvent::Result by its message otherwise, returning response unchanged only once the raised taint and the record are both on disk. Helpers not_recorded/audit_unavailable/denied; every append Err sets stopped through the one append method, and finish logs the raise error "brokerd: {e}" before stopping. Step 5 verified: each numbered exit points at a line and every append Err goes through the one stopped place. Trimmed 588 to 499 by compressing the module doc; one clippy fix (needless `return` in the answer append match, which is the tail expression). 11 + 9 tests pass; `make gate` prints `gate: ok`. | ? |
 | M3a/08-brokerd-state | 2026-09-18 | done | 1 | pass | none | Wrote crates/brokerd/src/state.rs: RUNBOOK, StateError (Unreadable/Write with hand-written Display ending in RUNBOOK and std::error::Error), StateStore (new does not touch disk, path joins <dir>/<id>.json, read, raise) and the private StateFile with deny_unknown_fields. read has exactly one default path (ErrorKind::NotFound); Public taint is Unreadable; raise computes max(taint,label,Private) and ORs untrusted, always writes atomically in six steps mapping any error to Write(path, err). `cargo fmt` put `state` after `runner` in lib.rs. 9 tests pass; `make gate` prints `gate: ok`. | Laguna S 2.1 |
 | M1/01-workspace-and-gate | 2026-09-17 | done | 1 | pass | none | Crate skeletons, Cargo files and the given Makefile/deny.toml/test-gate-scripts.sh were already present untracked from a prior attempt; I verified them against the plan and created only the missing gate scripts, dependencies.md, egress.md and this log row. | Laguna S 2.1 |
 | M1/02-proto-values | 2026-09-17 | done | 1 | pass | none | Implemented ValueError, SessionId, Epoch, CallId, Hash32 and Timestamp in crates/proto/src/ids.rs and DataClass in class.rs, using serde try_from/into for string-backed JSON validation, a hand-written hex encoder and humantime for RFC 3339 parsing with canonical re-serialization. | Laguna S 2.1 |