feat: add the worker-thread code runtime (dsh-code-runtime-worker)

The shipped backend of the code-execution seam, per the Code Mode RFC's
worker-thread section: one fresh Node worker per run, executing the
model's TypeScript after a host-side type-strip (wrapped in an
async-function shell so top-level return/await parse, sliced back out
position-preserved), bindings bridged over the message port under
hostile-peer rules (own-property name lookup, at-most-once replies,
post-settlement drops, null-prototype namespaces), logs streamed eagerly
with an in-band truncation marker, and two independent budgets — measured
event-loop busy time (computeMs) plus a never-pausing wall ceiling
(maxWallMs) — funneling into worker.terminate(). env: {} and execArgv: []
keep the isolate hermetic; disposal aborts in-flight runs and awaits
worker exits.

The worker entry loads unbuilt via Node's native type stripping
(src/worker.ts, erasable-only) and ships built as a sibling tsdown bundle
(lib/worker.js); tests/built-lib.e2e.ts pins the built load path under
plain node and joins the built-artifact smoke gate. Unit suites cover the
bootstrap in-process (fake port) and the runtime over real workers,
per-file 100%.

vitest.config.ts

@@ -31,7 +31,12 @@ export default defineConfig({
       // can't import one without booting it, so they are driven by the keyless
       // Loader-path smoke (a real subprocess) instead of the in-process unit
       // suite — the same reason `examples/start.ts` sat out of coverage scope.
-      exclude: ['packages/*/*/src/types.ts', 'packages/*/*/src/bin.ts'],
+      // `worker.ts` files are the same class as bin.ts: self-executing
+      // worker-thread entrypoints that only ever run inside a spawned isolate
+      // the v8 provider cannot observe. They stay thin glue over in-process-
+      // tested logic (bootstrap.ts) and are pinned by real-worker integration
+      // tests.
+      exclude: ['packages/*/*/src/types.ts', 'packages/*/*/src/bin.ts', 'packages/*/*/src/worker.ts'],
       // 100% or it doesn't merge (docs/testing.md: excessive tests are welcome).
       // Per-file so a well-covered big file can't subsidize a bare one.
       // Every v8 ignore comment must carry a reason — see the quality-gates RFC