Fix log viewer worker lifecycle

[trissim]

Summary

• restore LogFileLoader to the thread lifecycle declared by the log-viewer architecture
• retain interrupted workers until QThread.finished and reject stale signals by worker identity
• join loader and tailer workers through one idempotent cleanup authority
• remove the duplicate log-highlight parser from the legacy streamer

Root cause

LogFileLoader inherited QThread but overrode start() to launch an unparented QProcess. Switching logs called a zero-timeout wait and then replaced the loader, which destroyed the child process while it was still running. The race is especially visible during Windows process startup.

The loader now performs bounded tail loading in a real interruptible QThread. Superseded workers remain owned until completion, cannot update a newer selection, and are joined when the viewer closes.

Verification

• full pyqt-reactive suite: 210 passed
• deterministic 20-switch regression: correct final log and no destroyed process/thread diagnostics
• changed tests: Ruff passed
• changed production modules: py_compile passed
• git diff --check passed

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