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.

tsconfig.base.json

@@ -44,7 +44,8 @@
       "@deepseek-ai/dsh-llm-deepseek": ["./packages/llm-deepseek/src"],
       "@deepseek-ai/dsh-llm-pi-ai": ["./packages/llm-pi-ai/src"],
       "@deepseek-ai/dsh-bash-local": ["./packages/bash-local/src"],
-      "@deepseek-ai/dsh-tool-bash": ["./packages/tool-bash/src"]
+      "@deepseek-ai/dsh-tool-bash": ["./packages/tool-bash/src"],
+      "@deepseek-ai/dsh-invariants": ["./packages/invariants/src"]
     }
   }
 }