docs(architecture): rewrite the system map to the 1,800-word budget

architecture.md is the behavior map: layering, service map, seam
pattern, and the loop — everything else defers to its owning tier.

- Seam narrations compress to two-to-four sentences plus links to the
  RFC and type-catalog homes that carry the detail (turn-end variant
  semantics -> session.md, derivation mapping -> session.md, StreamChunk
  conventions -> llm-streaming.md + source).
- The MVP feature-to-mechanism checklist moves de-statused into the
  extension cookbook as 'The feature -> mechanism map' — mechanisms
  only, no implementation-status bolding to rot; the microkernel RFC's
  proof-obligation pointer follows it.
- The layering diagram describes layers by family instead of
  enumerating packages (the stale 'future plugins: hooks, compaction'
  row is gone); the dependency rule defers to packages/README.md.
- The loop pseudocode, the three externally-cited anchors (the
  vocabulary, event taxonomy, waterfall semantics), and the filename
  are unchanged.
- Budget ratchet: docs/architecture.md 3897 -> 1800 (now 1,797 words);
  the doc-tiers RFC's deferred list prunes the item this ships.

docs/rfc/implemented/architecture/2026-06-11-microkernel-event-taxonomy.md

@@ -20,7 +20,7 @@ The event vocabulary lives in interface packages (dsh-agent declares the agent/*
 
 ## Consequences
 
-- Every MVP feature maps to a listener (the "plugin sanity checklist" in docs/architecture.md is the proof obligation, kept current).
+- Every MVP feature maps to a listener (the [feature → mechanism map](../../../cookbook/extension-cookbook.md#the-feature--mechanism-map) is the proof obligation, kept current).
 - HMR and disposal come free: listeners and registrations are Cordis effects.
 - Waterfall semantics (call `next()` or short-circuit) are non-obvious and must be taught — documented in AGENTS.md and covered by composition tests.
 - The loop must be defensive: plugin exceptions are contained at turn level, steering from any seam is never stranded (regression-tested).