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/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) |