test: isolate Greg tee-output readiness window with LPI and CaPU

[safal207]

Summary

Adds a third, stacked evidence pass for gdb/tee-output#3 on top of PR #100. This pass tests the remaining startup/readiness candidate without promoting it to a sole root cause.

Exact upstream

gdb/tee-output@c41f8ff383200320b746e953e92709ae1b505a71

Counterfactual modes

CURRENT
SLEEP_100
FILE_EXISTS
SUPERVISOR_ACK
SUPERVISOR_ACK_SAFE_CLOSE

SUPERVISOR_ACK preserves the real system tee and parent-lifetime: a wrapper launches the actual tee, confirms that the child is alive and all output targets exist, and only then opens the CaPU-style write gate.

The acknowledgement is supervisory. It is not represented as an internal source-level acknowledgement from the system tee read loop.

Pinned components

LPI      0683ca3731256d5fc06f60c14a3deb9e9c149d19
CaPU     babd2945046d2564e1110a76741827560c57fcca
T-Trace  6a0755dbe8a89decd325298ec4563b9bb16adc62
TTM DB   eeded6bf5666512a23a7477eb8cda6485dfd173a
SDP      f1903aecb1f9196414c49784558f6ed9f40d8bda
DRP      92e63d7d4eeb55f8eb61956da002dc8951bab1c6

Interpretation boundary

• current fails + file_exists passes supports a pre-output-open startup window.
• file_exists fails + supervisor_ack passes supports an acknowledgement requirement beyond path existence.
• supervisor_ack fails keeps readiness insufficient.
• supervisor_ack passes + safe close improves further supports a multi-cause startup + shutdown model.

The final gate may permit a comment that readiness is supported but non-exclusive. It can never permit a claim that readiness is the sole root cause.

Authority

Read-only experiment. No third-party issue comments, state changes, labels, approvals, closes, or merges are performed by this PR.

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[safal207]

Exact-head readiness result

The clean final run completed on:

head: 070b7cb8d319fc87939ea979f1cf2ce5015a7c1c
run: 29779674997
artifact: 8476100343
artifact digest: sha256:60ee7ea0899e302e3d65afdd612a4bcb292f7dee74be8df211d446daa6164048

All platform, component, replay, T-Trace, TTM, SDP, DRP, and claim-boundary jobs passed.

Final machine verdict:

HOLD_READINESS_BARRIER_NOT_SUFFICIENT

Observed PTY profile in this exact attempt:

Mode	Linux	macOS
immediate current path	12/12 failed	12/12 failed
fixed 100 ms before write	0/12 failed	4/12 failed
target files exist	3/12 failed	1/12 failed
supervisor ack	0/12 failed	2/12 failed
supervisor ack + safe close	1/12 failed	2/12 failed

The two macOS supervisor-ack failures were both direct-fd burst cases. Startup acknowledgement succeeded, then the write burst completed and close() began in under 1 ms; only a tail of one stream was absent from its file and the combined file.

Therefore:

• elapsed time is not a reliable readiness state;
• file existence is not a reliable readiness state;
• supervisory readiness is strongly helpful but not sufficient;
• readiness cannot be claimed as the sole root cause;
• the next bounded candidate is a second phase: post-write drain/completion acknowledgement before close.

No upstream state was changed.

[safal207]

Superseded by PR #107, which carries the final exact-source gdb/tee-output#3 contributor candidate and cross-platform validation. This readiness-window investigation remains preserved as historical evidence but is no longer the active delivery path.