docs: architecture.md becomes the canonical target-architecture spec (stage 6)

Replaces the pre-pivot v0 scaffold doc (preserved in git history) with
the as-built spec: the five layers and their dependency directions, the
machine-enforced import law (four zones + the two documented-only
rules), ports/adapters and the two composition roots, the extraction
pipeline as sole graph writer (division of trust, merger invariants,
probe-attribution resolution, the v1 trigger seam), the pure read
channel, the turn seams, minimal observability, the testing strata, and
the deferred-with-links list. The doc states it describes the system,
not an intention — it changes with the commit that changes the system.

README reference updated (was: 'pre-pivot v0 scaffold — history only').

README.md

@@ -6,7 +6,7 @@ Not a chat app with a graph bolted on: sessions are probe→teach lessons, the p
 
 - Living design doc (all decisions): [`docs/vision.md`](docs/vision.md)
 - Build plan + wave/gate results: [`docs/plan.md`](docs/plan.md)
-- [`docs/architecture.md`](docs/architecture.md) is the **pre-pivot v0 scaffold** (pi-based, Fastify/Vite) — kept as history only; everything current is below.
+- [`docs/architecture.md`](docs/architecture.md) is the **canonical architecture spec** — layers, ports, the import law, the single-writer rule, the two graph channels.
 
 ## Commands