docs: make the service map the front door of architecture.md

The Layering ASCII diagram was cluttered, lumped THE concrete loop plugin
undifferentiated into a grab-bag plugins box, omitted dsh-agent-core and the
app packages, and restated the dependency rule that packages/README.md
already owns — while being the first thing a reader linked from README hits.

- Drop the Layering section; the tier story becomes one intro sentence and
  the dependency rule a one-line Service map footer linking
  packages/README.md#dependencies.
- Split the Service map into the spine (packages/core/) vs the swappable
  capability seams; annotate ctx.agentLoop as THE concrete loop plugin.
- Promote Cordis waterfall semantics to a top-level section placed before
  first waterfall use; drop '(important)' from the heading (anchor swept:
  AGENTS.md, gen-cordis-catalog.ts + regenerated catalog).
- Promote Event taxonomy to a top-level section (anchor unchanged).
- Retitle 'The vocabulary (dsh-llm)' to 'Content blocks and streaming
  (dsh-llm)' so the heading names its content (citation swept:
  llm-streaming.md).

Net -29 words (1851 -> 1822); the 1890 ceiling stands, keeping the
manifest's working margin.

scripts/gen-cordis-catalog.ts

@@ -545,7 +545,7 @@ function render(events: EventEntry[], services: ServiceEntry[]): string {
     '',
     '## Events',
     '',
-    'Dispatch modes: **emit** (fire-and-forget), **waterfall** (each listener gets `next()` and may transform or veto — see [waterfall semantics](../architecture.md#cordis-waterfall-semantics-important)), **parallel** (awaited fan-out; all listeners run), **serial** (awaited in registration order until one returns a bail value — anything other than `null`, `false`, or `undefined`).',
+    'Dispatch modes: **emit** (fire-and-forget), **waterfall** (each listener gets `next()` and may transform or veto — see [waterfall semantics](../architecture.md#cordis-waterfall-semantics)), **parallel** (awaited fan-out; all listeners run), **serial** (awaited in registration order until one returns a bail value — anything other than `null`, `false`, or `undefined`).',
     '',
   ]
   const scopes = [...new Set(events.map(e => e.scope))].sort()