feat(appservice): @mosaicstack/appservice — Matrix AS core (M4a, agent-comms#9)

First phase of mosaic-as per agent-comms docs/design/M4-MOSAIC-AS.md:
- TransactionHandler: AS transactions endpoint logic with timing-safe
  hs_token verification (Bearer + legacy access_token), bounded txnId
  dedupe (documented at-least-once limitation), handler error isolation
- AppserviceIntent: virtual user registration (m.login.application_service),
  user_id impersonation, invite-then-join fallback, thread-aware sends
  (m.thread + is_falling_back), typing, display names, uuid txn ids
- Registration builder + hardened YAML serializer (control-char rejection,
  quote escaping) for the Synapse mosaic-as.yaml
- bridge.dto.ts validators for the agent-comms daemon bridge API
Zero runtime deps; strict ESM/NodeNext; 14 vitest tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/appservice/tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src"
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist"]
+}