feat(invariants): dev-mode event-contract assertions + session-log freeze (RFC 005 pt 3, RFC 008)

New @deepseek-ai/dsh-invariants plugin (pure listeners, off in prod) asserts
the event taxonomy at runtime — seq monotonicity, turn/step nesting, a
tool/result needs a prior tool/call (NOT the converse), legal agent/status
transitions — and deep-freezes logged event data so mutating history throws.
Seeded sessions are checked + frozen on session/created.

The real RFC 008 fix is always-on: deriveMessages now structured-clones the
content it emits, so the loop's sanctioned request/adapter mutation can no
longer reach back and rewrite the append-only log. The pervasive
DeepReadonly<T> type flip is rejected (compile-only, high-noise, castable) —
recorded in ADR 0012, which folds in RFC 008. Wired into both demos.

docs/rfc/005-runtime-validation-and-error-taxonomy.md

@@ -1,6 +1,6 @@
 # RFC 005: Runtime validation at the model boundary, error taxonomy, dev-mode invariants
 
-Status: partially implemented — part 1 (arg validation) → [ADR 0011](../adr/0011-runtime-arg-validation.md); parts 2-3 in progress
+Status: partially implemented — part 1 (arg validation) → [ADR 0011](../adr/0011-runtime-arg-validation.md); part 3 (dev invariants) → [ADR 0012](../adr/0012-dev-invariants-over-deep-readonly.md); part 2 (error taxonomy) in progress
 
 ## Problem
 

docs/rfc/008-immutable-public-surfaces.md

@@ -1,6 +1,6 @@
 # RFC 008: Deep-readonly public surfaces
 
-Status: proposed
+Status: implemented (revised) — the pervasive `DeepReadonly<T>` type flip was rejected in favor of an always-on `deriveMessages` clone plus dev-mode `Object.freeze` + invariants. See [ADR 0012](../adr/0012-dev-invariants-over-deep-readonly.md).
 
 ## Problem
 

docs/rfc/README.md

@@ -11,4 +11,4 @@ Proposals for substantial future work — reviewed before implementation, unlike
 | [005](005-runtime-validation-and-error-taxonomy.md) | Runtime arg validation, structured error taxonomy, dev-mode invariants | partially implemented |
 | [006](006-doc-sync-and-api-reports.md) | Doc-sync enforcement and API extractor reports | proposed |
 | [007](007-supply-chain-and-vendor-drift.md) | Supply chain checks and vendor drift verification | proposed |
-| [008](008-immutable-public-surfaces.md) | Deep-readonly public surfaces | proposed |
+| [008](008-immutable-public-surfaces.md) | Deep-readonly public surfaces | implemented (revised) |