Preserve editor files when saving fails

[lukisch]

An interrupted editor save could truncate an existing file before returning an error. Invalid UTF-8 could also escape the save boundary after the target was opened. Both failures were reproduced on unchanged main at 4116f36.

EditorTab now encodes the full buffer as strict UTF-8 with native newlines before opening the target, then writes through Qt QSaveFile with direct-write fallback disabled. It checks open, byte count and final commit; on failure it cancels the temporary write, reports the error and keeps the document dirty. Existing SaveAs, Close and Run guards remain in place.

This branch also repairs CI issues exposed by the full suite: test jobs install Pillow; platform smoke tests use the selected interpreter and named debug argument and expose the terminal process error signal; a tomli fallback preserves metadata parsing on Python 3.10.

Validation:

• Current HEAD 56bf06b: all 13 GitHub Actions jobs passed, covering lint and Windows/Linux/macOS on Python 3.10–3.13.
• The unchanged 4116f36 full suite passed 346 tests with 1 skip. Candidate d528a85 passed 353 with 1 skip. Current HEAD is validated by CI; no separate final-head local full-suite run was started.
• 16 focused save tests and 69 tests against the final OneDrive overlay passed. Independent save review passed 25 cases; one native symlink case was skipped because Windows returned error 1314. Platform and metadata deltas received separate reviews.
• Hash readback covered all 173 tracked OneDrive paths. All 11 owned overlay files match this branch; 25 foreign deltas and the existing .git remain intact. The private config/settings.json was excluded from the isolated snapshot.

Version remains 0.3.5. No executable was built and no release was made. This remains Draft PR #2; merge still requires the separate review gate for a different model class.

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