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/adr/0012-dev-invariants-over-deep-readonly.md

@@ -0,0 +1,27 @@
+# ADR 0012: Dev-mode invariants over compile-time deep-readonly
+
+Status: accepted (2026-06-13)
+
+## Context
+
+The session log is append-only by contract, but the types don't enforce it: `session.events` returns `readonly SessionEvent[]` whose *elements* are mutable, and `deriveMessages()` handed the logged `content` arrays/blocks out by reference. The loop then passes those derived messages into the `agent/request` waterfall and on to adapters, where mutating the request is sanctioned — so a request middleware could reach back and rewrite history, silently breaking replay equivalence and the derived-history guarantee. Separately, the event taxonomy (turn/step nesting, seq monotonicity, tool-call/result pairing, legal status transitions) was asserted only where individual tests happened to look.
+
+Two ways to defend the log: make immutability part of the type (`DeepReadonly<SessionEvent>` on the way out), or catch corruption at runtime in dev. The RFC (005) proposed the runtime route; RFC 008 proposed the type route.
+
+## Decision
+
+Reject the pervasive `DeepReadonly<T>` type flip. Instead:
+
+1. **Always-on:** `deriveMessages()` deep-clones the content it emits (one `structuredClone` per derived message). In-flight mutation of a request can no longer reach the log — this is the real fix, and it costs nothing meaningful next to a model call.
+2. **Dev-mode:** a new `dsh-invariants` plugin (pure listeners, off in production, on in tests and demos) asserts the event contract and `Object.freeze`s logged event data so any *other* code that mutates a logged event throws instead of corrupting silently. Seeded sessions are frozen and checked on `session/created` (the constructor copies the seed without emitting `session/event`).
+
+The invariants encode the *real* contract, not an idealized one: a `tool/call` may have no `tool/result` (a thrown `tools/execute` waterfall ends the step), and both `idle→disposed` and `running→disposed` are legal.
+
+`DeepReadonly` was rejected because it is compile-time only (a plugin casts straight through it), high type-noise across every log/message consumer and adapter, and would force readonly types through code where mutation is the sanctioned API. The clone draws the mutable/immutable boundary exactly at "logged vs in-flight" without any of that noise.
+
+## Consequences
+
+- History corruption is caught loudly in tests and demos, at zero production cost and zero type noise. The trade-off is that the guarantee is dynamic (a dev-mode tripwire) rather than static.
+- The invariants plugin doubles as executable documentation of the event taxonomy — the assertions are the contract.
+- `Session.events` keeps its `readonly SessionEvent[]` type; no consumer churn.
+- This folds in RFC 008 — there is no separate deep-readonly ADR; this records the decision to *not* pursue that approach. `InvariantError` is a plain `Error` with a `code` for now; a later taxonomy change can promote it.

docs/adr/README.md

@@ -23,3 +23,4 @@ Do NOT write an ADR for: a mechanical or local choice (a variable name, a one-fi
 | [0009](0009-capability-seams.md) | Capability seams — interface / implementation / consumer split | accepted |
 | [0010](0010-twin-llm-adapters.md) | Two LLM adapters as a design-verification twin | accepted |
 | [0011](0011-runtime-arg-validation.md) | Runtime arg validation at the model boundary | accepted |
+| [0012](0012-dev-invariants-over-deep-readonly.md) | Dev-mode invariants over compile-time deep-readonly | accepted |