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.

scripts/doc-budgets.manifest.json

@@ -1,7 +1,7 @@
 {
   "AGENTS.md": 1500,
   "docs/AGENTS.md": 1250,
-  "docs/architecture.md": 3897,
+  "docs/architecture.md": 1800,
   "docs/defensive-patterns.md": 550,
   "docs/testing.md": 800,
   "examples/AGENTS.md": 579,