docs(code-runtime-python): align stale frame-ceiling prose with the 64 MiB parse cap; pin pythonBin resolution

The review's doc drift items: the orphan receive-ceiling JSDoc, the frame-ceiling
references in index.ts/bootstrap.py/tests, and the README's 'dropped, stranding
to the wall clock' phrasing (the run now settles as a worker-exit) are all
updated to the 64 MiB FRAME_PARSE_CAP_BYTES semantics; the README notes the
>64 MiB binding-argument residual as a worker-exit trip of the same cap. A
regression case resolves a basename pythonBin against a PATH whose first entry
is relative ('.') and asserts the absolute entry is used.

packages/code-runtime/code-runtime-python/py/bootstrap.py

@@ -2169,7 +2169,7 @@ def _cap_message(message: str, max_bytes: int) -> str:
     by raw UTF-8 length: the message crosses fd 3 inside a JSON frame where
     control characters escape up to sixfold (a NUL is one raw byte but six as
     ``\\u0000``), so a raw-length cap of ``maxValueBytes`` could serialize to
-    roughly six times that and breach the 256 MiB frame ceiling — the silent
+    roughly six times that and breach the 64 MiB frame parse cap — the silent
     ``worker-exit`` inversion the load-time cap check exists to prevent, and a
     several-hundred-MiB escape allocation besides. The seam's load bound admits
     ``maxValueBytes`` up to ``parse-cap - envelope`` on the premise that both the