chore(preview): delete six declarations nothing reads

[PathGao]

MarkdownViewer.svelte declares six top-level bindings the file never mentions again. They read as live state, so anyone tracing how rendering or scroll sync works has to prove each one is inert before ruling it out.

declaration	introduced	occurrences in the introducing commit
let containerEl: HTMLElement	b46a283	1
const renderDebounceMs = 50	b46a283	1
let renderTimeout: ReturnType<typeof setTimeout> | null	b46a283	1
let caretEl: HTMLElement	ef219cf	1
let caretAbsoluteTop = 0	ef219cf	1
const loadRevisionByTab = new Map<string, number>()	41de13b (#247)	4

Five were never used at all. One occurrence in the commit that introduces them means the declaration and nothing else. renderDebounceMs = 50 next to renderTimeout is the one most likely to mislead — together they describe a render debounce that does not exist anywhere in the file.

One is refactor residue. loadRevisionByTab had four occurrences when #247 added it here. bb8dbf3 (#274) extracted document loading into src/lib/sessions/documentSession.svelte.ts, which carries its own map and all three call sites; the declaration here stayed behind, dropping to one occurrence. Two identically named maps in two files, one of them dead, is worse than either alone — a reader who greps the name finds both.

How "unreferenced" was established

Grep alone is not sufficient in a Svelte component: bind:this={containerEl} in the template is a real use, and a name can be reached from the markup without appearing in the script block. So:

1. Counted occurrences across the whole file, template included — each is exactly 1.
2. Grepped src/ and scripts/ for each name outside this file — none.
3. npm run check (438 files, 0 errors) and npm run build — the compiler resolving the template is the check that matters. A template reference to a deleted binding fails the build.

scripts/largeFileLoadRevision.test.ts pins the live map in documentSession.svelte.ts and is untouched.

Verification

npm run check   438 files, 0 errors, 0 warnings
npm test        546 / 546
npm run build   clean

Not covered

• Only top-level let/const in this one file. Unused imports, unused function parameters, and dead code in other files were not swept — a wider sweep would be a different change with a different review surface.
• caretEl/caretAbsoluteTop came in with split view and scroll syncing (ef219cf). They were dead on arrival, so removing them cannot change scroll sync, but it also does not tell you whether the caret-tracking they were meant for is missing on purpose. That question is left where it was.

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