[KnifeLemon]

Refs #767 (the reporter still owes a log, so this doesn't close it)

What I measured

improper image header ... ReadPNGImage comes from ImageMagick. It means the first bytes of the file aren't a signature ImageMagick knows. With the real engine:

File named .png holding	Result
a PNG, a JPEG or a BMP	converts (the engine goes by content)
zeros, or zeros at the start	improper image header
text, or junk before a PNG	improper image header
half a PNG	unexpected end-of-file (another message)

A burst alone doesn't cause it. Real filee watch, files dropped with the Shell's MoveHere/CopyHere (the engine Explorer uses):

• 150 PNGs of 1.9 MB, moved: 150 done, all in originals. Copied: the same.
• 2500 small PNGs, moved, Keep mode: 2500 done. InternalBufferSize and the Error rescan lose nothing here, so "only 70" isn't a missed event as far as I can reproduce.

This doesn't reproduce the report. It closes what is really wrong in that path.

What was wrong

1. A file with no data yet counted as ready. A program can reserve a file's size, let go of it and write later. For those seconds the file keeps its size and time and opens without sharing. Reproduced end to end on main: failed: improper image header, then done. Now it waits for data at its start, for up to 30 s: some formats do start with zeros (ISO), and a damaged file must still get its error. It reads 4 KB, so DICOM's 128-byte preamble doesn't wait.
2. A failed file was never looked at again before it changed. _handled is filled before the conversion runs. It now gets one more turn after 10 s, per version of the file, so a broken file reports twice and then stays quiet. The cost: a broken file makes a second failed job in the toast, and a long video that fails at the end runs twice.
3. A batch that could not start was forgotten without a word (_convert returned null or threw). It gets the same one retry.

The change to FolderWatcher is small and additive, as a private branch edits other parts of that file.

Not changed

An overflow rescan in Keep mode also notices files that were already there and converts them again. Rare (it takes thousands of events), and unrelated to this report.

Tests

FolderWatcherTests: 7 new cases fail against main's FolderWatcher.cs. FolderWatcherTests, WatchRuleOptionsTests, WatchResultsTests, FolderCopyTests: 87 passed, 2 skipped. CLI watch tests: 8 passed. dotnet format --verify-no-changes clean on the changed files.

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