Enforce 100% per-file test coverage on packages/*/src

vitest coverage (v8 provider) with per-file 100% thresholds for
statements, branches, functions, and lines. Scope: our runtime source
only — types-only files, vendor/ (upstream code), and examples/
(exercised by the demo smoke test) are excluded. yarn test:coverage
runs the gate.

59 tests added to close every gap: llm generate-waterfall and adapter
disposal; assembler edge protocol (duplicate block-start, stragglers
after block-end, id fallback, usage omission, invariant violation);
the whole Inbox surface incl. the wakeup-overwrite race; LoopAgent
disposed-state throws and double-stop idempotence; config-driven agent
creation; loop backstop catches (throwing turn-start/turn-end
listeners, non-Error throws, non-JSON tool arguments); system-prompt
dynamic sections and disposer paths; tools errorMessage fallbacks and
the full schema-DSL emission matrix. Genuinely unreachable defensive
guards carry /* v8 ignore */ comments with stated reasons rather than
deletion (132 tests total).

package.json

@@ -17,11 +17,13 @@
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
     "test": "vitest run",
+    "test:coverage": "vitest run --coverage",
     "demo": "node --expose-internals --import tsx examples/echo-agent/start.ts"
   },
   "devDependencies": {
     "@stylistic/eslint-plugin": "^5.10.0",
     "@types/node": "^25.3.5",
+    "@vitest/coverage-v8": "^4.1.8",
     "dumble": "^0.2.3",
     "eslint": "^10.4.1",
     "tsx": "^4.22.4",