[KnifeLemon]

What changed

Krita keeps the previous save as name.kra~: the same KRA file under another name. Filee now reads it as KRA, so it converts like .kra (PNG, JPG, WebP, PDF, …) and the output is named name.png.

• kra~ is one more extension of the KRA entry in FormatRegistry, next to kra. Path.GetExtension("name.kra~") is .kra~, so detection, the images donut and the output name (NameWithoutExtension → name) all work without new code. This is the same thing the png_large / jpg_large aliases do.
• Schema 65: the images donut of existing installs learns kra~; a user's own donut that already claims it keeps it. (Schema 65 may be renumbered if another change takes it first: the .dbml and FLIC PRs add one each, and feat(tools): Add background tool #1081 just took 64.)
• Donut editor: an extension Filee knows is accepted as it is. Before, the ~ was refused, so the .kra~ the autocomplete offers couldn't be added back after removing it. Unknown text still has to be letters, digits and - _ + ..
• Watch folders skip names ending in ~. Krita writes the backup next to the drawing on every save; converting it too would put the previous picture under the same name.png. Before this change .kra~ was simply unknown, so watch folders behave as they did.
• docs/ENGINES.md: one line on the backup in the KRA paragraph.

Why not a general "trailing ~" fallback

A fallback in ExtensionOf/Detect that strips any trailing ~ would also make notes.txt~, report.docx~, config.ini~ (Emacs / gedit backups) convertible, change what ExtensionOf returns for every caller (donut matching, output names, watch folders, the format request), and would need its own rule for donut extensions. Only Krita's backup was asked for, and it is a byte-for-byte KRA, so one more extension is the narrowest correct change. If more ~ backups come up, the fallback can still be added then.

How it was tested

• FormatRegistryTests: sketch.kra~ is detected as kra.
• ImageExtensionAliasTests: .kra~ (any case) is KRA, named sketch, and opens the images donut together with .kra; notes.txt~ / sketch.kr~ are not detected; schema 64 → 65 learns kra~ once and leaves a custom claim alone.
• OutputPathResolverTests: sketch.kra~ → sketch.png.
• FolderWatcherTests: a watch folder ignores sketch.kra~.
• ExtensionInputTests: *.KRA~ → kra~; txt~ is still refused.
• KritaTests (engine): a generated sketch.kra~ converts to sketch.png from its merged picture.
• dotnet build Filee.slnx; all of Filee.Core.Tests (977 passed, 3 skipped, incl. FormatFieldsTests); KritaTests (5); ExtensionInputTests + LocalizationTests (250); dotnet format Filee.slnx --verify-no-changes.
• The whole Filee.App.Tests ran once before the rebase onto feat(tools): Add background tool #1081: 1826 passed, 6 skipped, 2 failed. MergeOrderTests passes on its own (a timing failure under load); RenderTests.Engines_page_shows_versions_descriptions_and_download_speed fails on this PC because a system Tesseract (C:\Program Files\Tesseract-OCR) reports no version; neither touches this change.

Closes #1077

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