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/gen-doc-graphs.ts

@@ -154,7 +154,7 @@ const SERVICE_ROLES: ServiceRole[] = [
     pkg: 'code-runtime',
     title: 'Code-execution seam',
     mode: 'seam',
-    implementations: [],
+    implementations: ['code-runtime-worker'],
     consumers: [],
     note: 'Runs one model-written program against host-provided async bindings; backends differ by substrate and language (the Code Mode RFC specifies the worker-thread backend and the tool-registry consumer).',
   },