test(ci): await complete workspace owner PID publication

[schlessera]

Closes #1513

What changed

The workspace-lease fixture publishes a newline-terminated PID and waits for the complete positive safe integer before treating entry as ready. Empty files and positive partial prefixes cannot authorize a liveness probe against PID 0 or an unrelated process. Every entry-marker consumer uses the readiness check, and the signal tests use its validated PID.

The associated work-issue guidance checks the actual pull merge ref before reconfirmation because GitHub can retain an older base across a draft/ready transition.

Why this shape

The fixture publication boundary was reproduced in #1513. The original historical SIGTERM failures did not record the probed PID; this scoped repair does not classify those particular events. Production helper behavior, cleanup assertions, offline guards and deadlines remain unchanged. This two-file repair is independently useful and is also included unchanged in Storage PR #2172.

Proof

Current candidate 02bbaf411e07873045f369a57c35deb581cee104, integrated base 498270fbdc4ef2788fdd3babb259b647d77f6c77, tree 92925a83db0c4daa07665b523898cca1be4718ca.

• Current full native lease suite passes 13 tests / 68 assertions, including both signals, real package exclusion and the pinned offline container wrapper.
• The original native publication control failed first at LEASE_EMPTY_PID_PUBLICATION_MUST_WAIT with PID 0. Independently accepting empty markers and unterminated positive prefixes makes the corresponding assertion fail; both pass after restoration. These earlier behavioral receipts retain their original source identities.
• Mutating actual helper writer admission to a shared lock makes the export-reader proof fail at PACKAGE_EXPORT_UNAVAILABLE; restoration passes. The production helper is unchanged in this PR.
• Current required bun run check:pr --base origin/main is failed: 955 fast tests pass, one fails, 6,323 assertions. Installed Vitest discovery hits its 15-second watchdog before JSON publication. Its structured receipt reports 15,229ms, timedOut: true, SIGKILL, zero stdout bytes and no inventory. Affected-test feedback did not start. The separate verification blocker is ci: file-only Vitest discovery reaches watchdog before inventory publication #2380.
• A focused observed discovery control passes 3 tests / 13 assertions on the unchanged candidate, with child exit 0 in approximately 11.74 seconds. It does not repair or replace the failed preflight. A bounded replay of the original fast-test predecessor sequence is diagnostic only.

The earlier hosted executions passed their selected categories but their aggregate failed as stale. Their older checkout/base remain historical evidence; no current complete hosted pass is claimed and this PR stays draft.

Checklist

• Required local preflight and complete selected hosted proof pass on the qualified candidate.
• The scoped fixture readiness repair is implemented and its native behavioral proof passes.
• PR and issue carry release 0.44.0; assignments will be read back before merge.
• No changeset: test fixture and workflow guidance only.
• Current preflight lint and leakage gates pass.
• Production helper, termination/exclusion assertions, offline guards and deadlines are preserved.

Anything left open

#2380 blocks the required local preflight. Selected hosted proof and actual squash/main verification remain required. The historical #2334 Storage bundle failure remains separate; independent delivery here does not qualify Storage or close that investigation.