test: property-based tests for protocol-shaped code (RFC 001)

Adds fast-check + one tests/properties.spec.ts per protocol-shaped package
(llm/BlockAssembler, session, tools/schema DSL, agent-loop scheduling). The
tools suite includes the RFC 001<->005 composition property (generated args
satisfying a spec pass validateArgs), closing the validator/InferArgs drift
risk from ADR 0011. Loop properties are deterministic (settle on agent/status,
no sleeps).

The BlockAssembler suite found a real bug on first run: 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) + regression test. Graduates RFC 001 -> ADR 0013.

packages/llm/src/assembler.ts

@@ -72,6 +72,11 @@ export class BlockAssembler {
       }
       case 'block-end': {
         const partial = this.ensure(chunk.index, chunk.block.type)
+        // First close wins: a second block-end for an already-closed index is
+        // a straggler (same rule as post-close deltas). Ignoring it keeps the
+        // streamed prefix and the final blocks() in agreement — otherwise a
+        // re-close could rewrite a block already flushed downstream.
+        if (partial.block) return
         partial.block = chunk.block
         return chunk.block
       }