chore: delete pre-fix copies that survived their own fixes

[PathGao]

A pure deletion — 186 lines — plus one guardrail so this cannot recur silently.

These are not idle leftovers

Four already-fixed behaviours each left a copy behind, and every copy looks like a reusable shared implementation. The danger is not that they take up space; it is that a future "the viewer is too big, let's use the shared version" reverts a merged fix, with no test going red.

Deleted	Zero-call evidence	How it had drifted
markdown.ts::renderRichContent (141 lines, exported)	grep -rn "utils/markdown" src scripts src-tauri/src — only processMarkdownHtml is ever imported	Missing rememberDiagramSource. Without that attribute, the light-theme Mermaid re-render that PDF/HTML export depends on (#359) finds nothing to restore. It also hand-rolls isDark ? "dark" : "neutral" instead of resolveMermaidTheme — whose own comment says it exists "so the two cannot drift" — and carries an inline DOMPurify config instead of the shared contract
markdown.ts::getLanguage	one definition, no importer; the viewer calls its own copy	—
markdown.ts::highlightColorMap	same	—
Editor.svelte::getEditorScrollMax	one hit in the whole tree: the definition	Pre-#316 version using getScrollHeight(); the live getEditorContentScrollMax uses getContentHeight(). Reviving it re-introduces the editor's bottom padding into split scroll sync
Editor.svelte::getViewState, ::revealLine	component exports with no caller; the viewer's editorPane type declares neither	—

isYoutubeLink / getYoutubeId are kept — still called inside processMarkdownHtml — but their now-pointless export is dropped.

Why nothing caught them

youtubeExternalFallback.test.ts, taskToggleMemory.test.ts, mermaidPrintTheme.test.ts and previewScrollSync.test.ts each readFileSync one hard-coded path. A second copy living anywhere else is invisible to all of them.

scripts/singleImplementationConvention.test.ts generalises the allowlist idea already used by renderPipelineConvention.test.ts: a data-driven table where each row is { marker, allowed paths, optional requires }, scanned across the whole src tree. Adding a newly-fixed behaviour is a one-row append. Eight rules ship, including:

• mermaid.render( may appear only in MarkdownViewer.svelte / mermaidPrint.ts, and each such file must also contain rememberDiagramSource — that is fix(print): keep PDF exports readable in all themes #359 pinned
• getScrollHeight() - …height is allowed nowhere — Exclude editor bottom padding from split scroll sync #316 pinned
• createElement('iframe'), from 'dompurify', renderRichContent, getLanguage, highlightColorMap each pinned to their one legitimate owner

Plus a meta-test asserting every rule still matches something, so a renamed symbol cannot turn a rule into a silent no-op.

Negative control: with the two edited source files stashed, 7 of the 9 tests fail, then pass again. It catches precisely the copies this PR removes.

One unrelated fix rode along

Toc.svelte's scroll $effect returned a cleanup that called removeEventListener on the current value of a prop rather than the node it had attached to. The element is stable today so it does not misbehave, but if the prop ever changed the listener would leak, and if it became null the cleanup would throw. Now captures the node.

Checks

npm run check   432 files, 0 errors, 0 warnings
npm test        391 / 391
cargo test       98 / 98

No test lost coverage — none of the deleted symbols had a test referencing it, and previewScrollSync.test.ts pins the live getEditorContentScrollMax, untouched.

One line in another test: #384's DOMPurify.sanitize call-site allowlist listed markdown.ts, whose only call site was inside the dead renderRichContent. That allowlist must be exactly consumed, so deleting the call site correctly failed it. The entry is removed. This is the allowlist doing its job, not a workaround.

Not deleted, and why

• onDestroy in Editor.svelte — it is used; an earlier scan claimed otherwise and was wrong.
• exportHtml.ts::escapeHtmlText, a zero-increment return escapeHtml(v) wrapper — it still has a live caller in export.ts. Worth inlining separately.
• Editor.svelte's getValue / setValue / focus / restoreViewState component exports also appear to have no callers (every in-file hit is on the Monaco object, not the component export). Left alone to keep this a scoped deletion; worth a follow-up.
• A live pre-fix copy in MarkdownViewer.svelte — replaceWithYoutubeEmbed builds the very <iframe src="youtube.com/embed/…"> that a merged fix replaced with a browser-opening thumbnail, and frame-src is gone from the CSP so it could not even load. It is zero-caller, and youtubeExternalFallback.test.ts asserts no iframe — but only against markdown.ts, so this copy was invisible: exactly the failure mode this PR exists to close. It is handled in refactor(preview): drop the dead YouTube iframe copy from the viewer #388; until that lands, the iframe rule ships with that one path allowlisted under a KNOWN STALE COPY comment telling the next person to delete the copy and the allowlist entry together.

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