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).

packages/llm/src/assembler.ts

@@ -131,6 +131,7 @@ export class BlockAssembler {
     const ready: ContentBlock[] = []
     while (this.flushed < this.order.length) {
       const index = this.order[this.flushed]
+      /* v8 ignore next 3 -- noUncheckedIndexedAccess guard: loop condition guarantees index exists in a non-empty array */
       if (index === undefined) break
       const partial = this.mustGet(index)
       if (!partial.block) break
@@ -150,6 +151,7 @@ export class BlockAssembler {
     const remaining: ContentBlock[] = []
     while (this.flushed < this.order.length) {
       const index = this.order[this.flushed]
+      /* v8 ignore next 3 -- noUncheckedIndexedAccess guard: loop condition guarantees index exists */
       if (index === undefined) break
       remaining.push(this.assemble(this.mustGet(index), index))
       this.flushed += 1