[PathGao]

Follow-up to #1030. Most cases in the *.checks.* files read source as text: they sliced function bodies out of components, cut code out of .svelte files and evaluated it, or matched a line's spelling. This rewrites them as tests that run the code.

Logic a test could not reach is moved out of the components into modules the components call (41 new files under src/lib: the viewer's window exit, external change, view mode, viewer command table, preview links and copy; the editor's actions, key handlers, clipboard, mount sequence, document readings and jumps; the settings panes as row data; and similar). Each move is the old code plus its call site, with no intended behavior change. A few Rust functions became pure or generic over Runtime so they can be unit tested, with the tauri test feature as a dev-dependency only.

Every rewritten test was checked by putting the old bug back (or breaking the guarded behavior) and watching it fail for that reason. Small components without Monaco are now mounted under jsdom for their markup wiring; AGENTS.md is updated to say so and to stop asking for source-shape assertions. The checks files keep 177 cases: Tauri command/event names, config and i18n keys, and single-implementation conventions, which a follow-up moves to generated bindings and lint rules.

Order-dependent tests (pre-existing in the external change reload spec) are fixed; vitest --sequence.shuffle and node --test --test-random-seed pass on 10 seeds each.

These 18 checks are deleted with no replacement, because they need MarkdownViewer.svelte or Editor.svelte mounted, or real CSS layout:

• "saveContent is the choke point auto-save and the close dialogs share" (documents.checks.test.ts): the viewer's auto-save timer writes through saveSilently, so a file that decoded with losses is never auto-saved over itself.
• "the conflict bar offers Compare beside the two answers" (documents.checks.test.ts): when the file changes on disk, the bar shows Compare next to Keep mine and Reload, so you can look before you pick the answer that cannot be undone.
• "the viewer hands over an article rather than its own preview" (export.checks.test.ts): PDF export prints a dedicated <article id="print-root">, not the live preview with its folds and theme.
• "the preview scroll and the outline are wired to the view mode" (layout.checks.test.ts): scrolling the preview moves the outline highlight.
• "the outline's toggle drops the editor toolbar's offset once it is over the outline" (layout.checks.test.ts): when the outline is open, its toggle button doesn't sit on the outline's first entry.
• "every renderer line reaching the editor goes through the shift" (navigation.checks.test.ts): clicking an outline heading in a file with front matter opens the editor on the right line.
• "inline code in a heading scales with the heading" (preview.checks.test.ts): code inside a ## heading is sized relative to the heading, not stuck at the code font size.
• "the setting still governs code everywhere else" (preview.checks.test.ts): the Code Font Size setting still sizes code outside headings.
• "the document-level copy handler is attached" (preview.checks.test.ts): ⌘C and Edit ▸ Copy in the preview go through the app's copy handler (plain text plus text/html, no WebArchive).
• "release notes can be selected even though the app disables selection" (release.checks.test.ts): you can select and copy text in the update dialog's release notes (Makes the release note section better #532).
• "an inactive row is dimmed rather than hidden" (settings.checks.test.ts): a setting that has no effect right now (for example Wrap Column with wrapping off) is greyed out, not removed.
• "the auto-save debounce still reads the one switch" (settings.checks.test.ts): the auto-save timer only runs when Auto-save is on.
• "the panel's groups are visibly separated, not run together" (shortcuts.checks.test.ts): the Shortcuts pane draws a themed line and space between groups.
• "the viewer wires the editor's oncommand to runViewerCommand" (shortcuts.checks.test.ts): ⌘O and ⌘T typed inside the editor do the same thing as in the preview.
• "the preview holds one document host per tab, not one for all of them" (tabs.checks.test.ts): switching tabs doesn't rebuild the whole preview or lose your reading position.
• "startup restore reads content from disk, not from the snapshot" (windows.checks.test.ts): on launch, restored tabs read their text from disk.
• "the scope line is drawn on the strip, in a channel the active tab does not use" (windows.tagEditor.checks.test.ts): a window tag's coloured line runs along the bottom of the tab strip and can't be mistaken for the active-tab marker.
• "the status bar keeps its items on one line" / "the status bar clips what does not fit rather than growing" (editor.checks.test.ts): the status bar stays one line and clips instead of wrapping or growing, so a selection in a narrow window does not end the drag (Shift + Alt + LMB drag in the editor takes into account the window size #762).

Verified: node --test, vitest (normal and shuffled), svelte-check, cargo fmt/clippy/test, npm audit all green. Not clicked through in a built app.