Add RFCs for the remaining quality-proposal ideas

Eight proposals grouped by category, each with problem statement,
concrete plan, and risks: property-based testing over the
protocol-shaped core (chunk streams, event logs, schema DSL);
mutation testing as the counterweight to the 100%-coverage gate;
deterministic tests + a universal replay-invariant fixture + nightly
race stress; architectural conformance (dependency-cruiser rules and
the LlmAdapter conformance kit); runtime arg validation at the model
boundary with a structured error taxonomy and dev-mode invariants;
doc-sync enforcement (typechecked doc snippets, API reports);
supply-chain checks and nightly vendor-drift verification against the
manifest; and deep-readonly public surfaces (logged-vs-in-flight
mutability boundary). AGENTS.md points at docs/adr and docs/rfc.

docs/rfc/002-mutation-testing.md

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+# RFC 002: Mutation testing as the coverage counterweight
+
+Status: proposed
+
+## Problem
+
+The per-file 100% coverage gate (ADR 0007) proves every line *executes* under
+test — not that any assertion would notice if the line were wrong. Under
+agent-written tests, coverage pressure can produce execution-without-assertion.
+Mutation testing measures what coverage cannot: whether the suite *kills*
+deliberately injected bugs.
+
+## Proposal
+
+Stryker (`@stryker-mutator/vitest-runner`) over `packages/*/src`:
+
+- **PR-scoped incremental runs** (changed files only) as a CI job — fast
+  enough to gate merges once tuned.
+- **Nightly full runs** with a tracked mutation score; start by recording,
+  then set the threshold at the observed baseline and ratchet upward (same
+  policy as coverage: thresholds only ever tighten).
+- Surviving mutants are work items: an agent picks a survivor, writes the
+  killing test, repeats — a well-shaped autonomous loop.
+- Equivalent mutants (provably behavior-preserving) get annotated exclusions
+  with reasons, mirroring the `/* v8 ignore */` policy.
+
+## Plan
+
+1. Add Stryker config scoped to one package (llm — smallest, most algorithmic)
+   and measure runtime.
+2. Expand to all packages; record baseline scores in the config.
+3. Wire the nightly job; add the incremental PR job once runtime is acceptable.
+
+## Risks
+
+Runtime: mutation testing is expensive; per-file 100% coverage helps (every
+mutant is at least reached). If PR-scoped runs stay too slow, keep them
+nightly-only and rely on the score ratchet.