fix(code-runtime-python): send the run frame after boot-ack; reject directories in pythonBin resolution

The review's two behavior items: the run frame was written back-to-back with
the boot frame (the seam contract puts run after boot-ack, which confirms the
namespaces were accepted); it now goes out from the boot-ack handler, so a
boot failure cannot race the run frame. resolvePythonBin now requires the
candidate to be a regular file — a directory passes X_OK and would otherwise
shadow a later real interpreter. Doc spots: the load-time overflow message
says worker-exit (not stranding to the wall clock), the run JSDoc spells out
the resolve-with-error contract, the PATH-stub test removes the stale v8
ignore, and the README's binding-value bullet names serialization cost.

packages/code-runtime/code-runtime-python/README.md

@@ -116,7 +116,7 @@ These limits define what the package does and does not cover; they are current p
 - **An fd-3 frame whose raw length exceeds 64 MiB settles the run as a worker-exit** — `maxLogBytes`/`maxValueBytes` are load-bounded to the same parser cap so an honest child's frames always fit; a model-constructed binding ARGUMENT above 64 MiB (a value with no seam-level budget) trips the same cap — an accepted residual of the OOM guard.
 - **A combined log-and-value peak is not modelled by the load gate** — a model daemon thread that keeps writing while the completion value is metered and framed can add the two peaks in a way no gate admits or rejects; the run dies as `worker-exit`, containment holds, and only the failure classification is degraded.
 - **A 1-second dual-limit `ulimit -t 1` CPU overrun is reported as `worker-exit`, not a timeout** — when the host starts under a hard CPU limit equal to the soft and that limit is 1, `_clamped` cannot lower the soft, so the kernel SIGKILLs the busy loop and SIGXCPU is never delivered; containment holds, only the classification is degraded.
-- **No byte cap on intermediate binding values** — the implementation remains bounded by structured-clone cost and process memory, and a provider or executor may apply its own fetch cap.
+- **No byte cap on intermediate binding values** — the implementation remains bounded by the lossless-JSON serialization cost and process memory, and a provider or executor may apply its own fetch cap.
 
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 ### Dev Note