Cut redundant per-file scans while streaming Rapid Diffs

What does this MR do and why?

Several per-file jobs scanned far more than they needed while diff files streamed in: line links were rewritten for every mounted file even though only visible files need it, the page-wide lazy-image observer rescanned the whole document on every mutation batch, the viewed-state lookup queried the whole document per file, and each adapter rebuilt its context on every event. This MR scopes line-link rewriting to visible files, pauses the lazy-image observer during streaming and rescans once afterwards, scopes the viewed-state lookup to the document head, and caches the adapter context. No UI change.

Measurements

Local 1,000-file merge request (998 streamed files, about 85k elements), Overview tab then Changes tab, headless 1440x900, no extensions, master and the branch served by the same local stack, 3 runs each, medians.

Browser	Blocked before	Blocked after	Longest task before	Longest task after	Last file before	Last file after
Chromium 151	748 ms	798 ms (within noise)	319 ms	352 ms (within noise)	5.0 s	5.0 s (within noise)
Firefox 152	2,518 ms	2,176 ms	1,024 ms	996 ms	5.7 s	5.7 s (within noise)

On its own this is a modest Firefox-only win. It mostly removes wasted work that is more visible once the larger costs addressed in !253736 (merged) and !253645 are gone.

Reproduction steps

1. Open a large MR that includes images in the diff.
2. Go to Overview, then click the Changes tab.
3. Confirm lazy images still load after streaming finishes.
4. Confirm line links still work on visible files: click a line number and check that the URL updates.
5. Confirm the viewed checkbox state still renders correctly.

References

Split out of !253645 (original umbrella MR, now the complete-file streaming MR).

Sibling MRs from the same investigation:

• !253645 - Insert complete diff files while streaming Rapid Diffs
• !253736 (merged) - Stop re-rendering the file browser per streamed diff file
• !253738 (closed) - Skip the web streams polyfill when the browser can pipe natively (closed, no measurable effect on Rapid Diffs)
• !253739 (closed) - Keep streamed HTML tasks short in the chunk writer (closed, no measurable effect on Rapid Diffs)
• !253740 (closed) - Render identical diff file header buttons once per request (closed, no measurable effect on Rapid Diffs)

Feedback thread: #596236

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