feat(code-runtime-python): add the CPython subprocess backend

Land the PythonCodeRuntime implementation on top of the fd-3 protocol
seam: python3 -I per run, binding namespace over fd 3, RLIMIT_CPU/AS,
wall-clock timer, and SIGTERM->grace->SIGKILL process-group teardown,
with the real-subprocess integration suite.

Fixes three defects surfaced on the source PR's review before they ship:
- boot-write failure resolved a worker-exit through finish()/settle()
  that read wallTimer/onAbort/live in their TDZ, rejecting run() instead;
  the boot write now runs after those bindings and the v8-ignore that hid
  the branch is removed.
- log capture serialized against settlement with no lock while model
  daemon threads keep writing; LogBuffer now owns one shared re-entrant
  lock taken by write/flush_line/push.
- the fd-3 line residual was a subarray view pinning the whole joined
  frame; it is copied into a right-sized Buffer via detachResidual so
  pendingBytes measures what is retained.

packages/code-runtime/code-runtime-python/package.json

@@ -33,10 +33,17 @@
   "license": "MIT",
   "peerDependencies": {
     "@deepseek-ai/dsh-invariants": "workspace:^",
+    "@deepseek-ai/dsh-session": "workspace:^",
+    "@deepseek-ai/dsh-timeout": "workspace:^",
     "@deepseek-ai/cordis": "workspace:^"
   },
+  "dependencies": {
+    "@deepseek-ai/schemastery": "workspace:^"
+  },
   "devDependencies": {
     "@deepseek-ai/dsh-invariants": "workspace:^",
+    "@deepseek-ai/dsh-session": "workspace:^",
+    "@deepseek-ai/dsh-timeout": "workspace:^",
     "@deepseek-ai/cordis": "workspace:^"
   }
 }