Problem

The CPython subprocess backend for PTC mode, built on the fd-3 frame protocol, resolves every program outcome as a CodeRunResult, rejects run() only for seam misuse, and disposes to quiescence so no subprocess that stays in the child's process group outlives the fiber (a descendant that escapes the group with setsid() is the documented exception — see the package README's Known Limitations). A sequence of review passes surfaced defects that broke those contracts in ways unit coverage did not catch — each hid behind a /* v8 ignore */, a captured-callable that read as a fix but was not, a memory effect invisible through the seam, a load-time bound that double-counted, a process-group escalation that a survivor could outlast, a cross-event-loop completion that silently deadlocked, a synchronous throw outside the settlement path, or a transport boundary rendered as a log boundary. Most behavioral fixes ship with a test that fails without them; eleven do not, and are called out as such — the chunked frame read (a syscall-count improvement with no cross-platform-deterministic failure), the confirmed-empty finalize (its only seam-observable effect, a frozen heartbeat, freezes the instant SIGKILL is delivered, which the pre-fix finalize-on-delivery code also produced, and the discriminating probe is the signal-0 check the Alternatives reject as cross-environment-unreliable), the shared stdout/stderr budget (its only seam-observable difference is which entry boundary a mid-stream flush lands on, and that depends on the relative arrival timing of two independent OS pipes, which os.sched_yield does not make deterministic; the per-pipe memory bounds it strengthens ARE covered by the single-pipe flood tests), the flush_line join-clear-push reorder (it lowers the settlement-flush peak from three copies to two, but the 12x load gate already covers the three-copy newline path, so every gate-admitted config stays within the address space under both orders and no seam-observable difference exists — the memory effect is inside the Python child, unmeasurable through the seam like the shared-budget case), pacing binding replies (its in-tree case only asserts the framed replies still round-trip; the peak it removes lives inside the host's fd-3 writable buffer, unseen through the seam, so the 32.0 MiB → 0.0 MiB reduction is measurable only out-of-tree), dropping a late binding resolution before snapshot (its three assertions all also hold pre-fix, because sendReply already dropped after-settlement values — just later than the snapshot), the done-value TOCTOU pre-encoding (a concurrent mutation racing the encode cannot be deterministically constructed through the seam — its daemon-mutation regression only asserts the result is never a worker-exit, which is probabilistic and non-discriminating, so under the existing no-fail-before-with-a-reason precedent it is registered as no-fail-before), the stray-UTF-8 budget-flush retention (a budget flush landing exactly on a multibyte boundary is not schedulable through the seam; it is cross-referenced as v8-ignored), and the late-rejection settled guard (a rejection arriving after the run has already settled cannot be deterministically constructed from the seam), and the log-fragment seal (a 25 M single-character drip that would OOM is not deterministically constructible in CI; the in-tree case only asserts it completes and truncates), and the unknown-binding preview cap (the whole-target JSON.stringify peak is a transient allocation inside the reply path — its only seam-observable trace is peak memory under a forged near-ceiling global/name, not measurable through the seam; the in-tree case only asserts the run completes).