Failed writes of session state and event logs are ignored, and a corrupt sessions.json starts empty

[ParallelEntrepreneur]

What is wrong

Writes that failed are silently ignored, so what the UI shows can differ from what is on disk.

• persist_sessions ignores a failed write of sessions.json.tmp, and ignores a failed rename over sessions.json (crates/colonizer/src/sessions.rs:253-262).
• append_line, used for the events and logs of every colony, discards a failed write_all (sessions.rs:290-292).
• On startup, a sessions.json that can't be read or parsed becomes an empty list (crates/colonizer/src/main.rs:335). Every colony then disappears from the UI, although its worktree, branch and microVM may still exist. The next persist overwrites the corrupt file, so it can't be recovered either.

After a failed write, colonies keep running and the UI keeps showing progress, but the event log, the evidence of what happened, can have gaps. Nobody is told.

What done looks like

• A failed persist or append is logged loudly and visible in the UI. The mothership doesn't report success until the write is confirmed.
• A corrupt sessions.json is moved aside, for example to sessions.json.corrupt-<time>, instead of being overwritten, and the UI says so.
• Tests inject EIO, ENOSPC, permission-denied and rename failures.

Source: external audit of v0.1.3 (d89ce76), finding F08. Confirmed against the code.

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_{Credited to @colonizer-settlers as co-author on this issue.}