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/001-property-based-testing.md

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+# RFC 001: Property-based testing for protocol-shaped code
+
+Status: proposed
+
+## Problem
+
+Example-based tests pin the cases we thought of. The harness's core is
+protocol-shaped — chunk streams, event logs, schema conversion — where the
+input space is combinatorial and the interesting bugs live in interleavings
+nobody wrote an example for (the `streamBlocks` ordering bug survived 100%
+line coverage of the happy paths).
+
+## Proposal
+
+Adopt fast-check (vitest integration) with generators for our vocabulary:
+
+- **BlockAssembler**: arbitrary chunk sequences (valid and malformed —
+  duplicate indices, stragglers after block-end, missing block-start).
+  Invariants: `flushReady() + flushRemaining() ≡ blocks()` in order;
+  `streamBlocks ≡ generate().message.content`; memory bounded (partials map
+  size ≤ distinct indices); idempotent re-assembly.
+- **Session**: arbitrary event logs (seeded generators over SessionEventMap).
+  Invariants: `deriveMessages` deterministic; replay-from-seed produces
+  identical derivation; seq strictly monotonic; derived history unaffected by
+  non-message events.
+- **Schema DSL**: arbitrary SchemaSpecs. Invariants: generated JSON Schema's
+  `required` array equals the `required: true` keys at every nesting level;
+  conversion is total (never throws); generated args satisfying `InferArgs`
+  validate against the generated schema (once RFC 005's validator exists —
+  the two RFCs compose).
+- **Inbox/loop**: arbitrary send/steer/abort schedules against a scripted
+  adapter. Invariants: no message lost (every send/steer appears in the log
+  exactly once), turn numbers strictly increase, status transitions follow
+  idle→running→idle/disposed.
+
+## Plan
+
+One `tests/properties.spec.ts` per package; fast-check as devDependency;
+numRuns tuned so the suite stays under ~10s locally, with a nightly CI job
+running 100× the iterations. Failures persist their seed in the report so
+agents can reproduce deterministically.
+
+## Risks
+
+Generator quality determines value — invest in generators that produce
+*realistic-but-adversarial* streams, not uniform noise. Property flake from
+timeouts must be treated as a finding, not retried away.