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%.

scripts/check-workspace-constraints.ts

@@ -105,11 +105,26 @@ const dshBinPackageFiles = [
   'src',
 ] as const
 
+// Packages that ship a worker-thread entry as a sibling runtime bundle
+// (lib/worker.js, its own tsdown entry): the bootstrap is loaded via
+// `new Worker(new URL('./worker.js', import.meta.url))`, so it cannot live
+// inside the index bundle and must be published alongside it.
+const workerEntryPackages = new Set(['@deepseek-ai/dsh-code-runtime-worker'])
+
+const dshWorkerPackageFiles = [
+  'lib/index.js',
+  'lib/worker.js',
+  'lib/types/**/*.d.ts',
+  'lib/types/**/*.d.ts.map',
+  'src',
+] as const
+
 function sameStringList(actual: readonly string[] | undefined, expected: readonly string[]): boolean {
   return !!actual && actual.length === expected.length && actual.every((value, index) => value === expected[index])
 }
 
 function expectedDshPackageFiles(manifest: PackageManifest): readonly string[] {
+  if (manifest.name && workerEntryPackages.has(manifest.name)) return dshWorkerPackageFiles
   return manifest.bin ? dshBinPackageFiles : dshPackageFiles
 }