docs: unify ADR/RFC trees into one lifecycle-organized RFC tree

Collapse docs/adr/ and docs/rfc/ into a single docs/rfc/ with proposed/,
implemented/, and rejected/ subfolders. Every file is renamed to
yyyy-mm-dd-topic-title.md, where the date is when the topic was first
proposed (from git history). ADRs and RFCs that covered exactly the same
topic are merged (property-based testing, session persistence); the
umbrella RFC 005 stays split across its three implemented decisions, and
RFC 006's deferred part-3 (API extractor reports) splits into its own
proposed RFC. All cross-references become machine-checkable relative
links instead of bare "ADR NNNN" / "RFC NNN" prose.

Add a verify-md-links doc-sync gate (scripts/verify-md-links.ts) that
checks every relative Markdown cross-link resolves, wired into doc-sync
alongside verify-md-wrap. This makes the reorganization self-verifying:
the same change that rewrote ~forty inter-doc links adds the check that
proves none dangle. Document the cross-link convention in a new
docs/AGENTS.md and record the gate as an implemented RFC.

doc-sync, typecheck, lint, and the full test suite (667) all pass.

docs/rfc/implemented/2026-06-11-property-based-testing.md

@@ -0,0 +1,27 @@
+# RFC: Property-based testing for protocol-shaped code
+
+Status: implemented (proposed 2026-06-11, accepted 2026-06-14)
+
+<!-- XXX: legacy ADR/RFC body format, not yet normalized to a unified RFC template. -->
+
+> Merges the original proposal and the decision record for one topic. It found a real BlockAssembler duplicate-`block-end` bug on first run.
+
+## Context
+
+Example-based tests pin the cases we thought of. The harness's core is protocol-shaped — chunk streams, event logs, schema conversion, inbox scheduling — where the input space is combinatorial and the interesting bugs live in interleavings nobody wrote an example for. The motivating evidence: a `streamBlocks` ordering bug once survived 100% line coverage of the happy paths. Per-file 100% coverage proves every line ran, not that every interleaving is correct.
+
+## Decision
+
+Adopt `fast-check` (a root devDependency) with one `tests/properties.spec.ts` per protocol-shaped package, generators tuned for *realistic-but-adversarial* inputs (not uniform noise) and `numRuns` kept so the suite stays well under ~10s locally, with a nightly CI job running 100× the iterations. Failures print a reproducible seed.
+
+- **dsh-llm / BlockAssembler:** arbitrary chunk streams (valid + malformed: duplicate indices, stragglers, missing block-start). Invariants: `flushReady()+flushRemaining() ≡ blocks()` in order; the streamed prefix is always a prefix of final `blocks()`; partial count ≤ distinct indices; re-assembly idempotent.
+- **dsh-session:** arbitrary event logs. Invariants: `deriveMessages` deterministic; replay-from-seed identical; seq strictly monotonic; non-message events never affect derived history; derived content is decoupled from the log.
+- **dsh-tools:** arbitrary `SchemaSpec`. Invariants: JSON Schema `required` equals the `required:true` keys at every level; conversion total; **and the composition with [runtime arg validation](2026-06-11-runtime-arg-validation.md)** — generated args satisfying a spec pass `validateArgs`, and targeted corruptions (dropped required key, non-object top level) are rejected. This closes the validator/`InferArgs` drift risk.
+- **dsh-agent-loop:** arbitrary send schedules against a never-exhausting adapter, driven through the `agent/status` settle signal (no wall-clock sleeps). Invariants: no message lost; turn numbers strictly increase; status transitions stay on the legal machine.
+
+## Consequences
+
+- Generator quality is the value lever — the generators bias toward small index pools and short strings so collisions and interleavings are common.
+- **It already paid off:** the BlockAssembler stream found a real bug — a duplicate `block-end` at the same index overwrote an already-flushed block, so the streamed prefix disagreed with final `blocks()`. Fixed (first close wins, matching the existing straggler rule) with a dedicated regression test.
+- A property flake from a timeout is a finding, not something to retry away. The loop properties are deterministic by construction (settle on `agent/status`), so a hang is a real defect.
+- Property tests supplement, not replace, the example tests that pin specific branches for the 100%-coverage gate.