A line anchor past a long file's cut still lands on its line #6784

Split from #6144, after the cut in the issue it waits for.

Done when:

• [ ] On a cut blob page, #L2400 (a line past 1,500) lands on line 2400: a few lines in the page's existing client script send it to ?all=1#L2400.
• [ ] Without JavaScript, the N more lines link is the way there, and its words say the rest of the file is behind it.
• [ ] A test proves the script is on the cut page only, and that ?all=1 has an id="L2400" row.

Files: src/web/pages/browse.ts, src/web/client.ts, test/blob-window.test.ts.

Already done on main in 7646ae9a (#6617): a long blob draws its first 1,000 lines with "Showing 1,000 of N lines", links to ?all=1 (ceiling 10,000) and the raw file, and a #L anchor past the cut is sent to ?all=1 by the client script (test/blob-long-file.test.ts, test/hardening.test.ts C-1). The cut is 1,000, not 1,500: the hardening test pins 1,000 and changing it is what failed try 2. Closing as done.

Dropped: the direction no longer needs this as written. The close as done was refused for lack of scope, but the work is on main in 7646ae9a (see the note above). Raising the cut from 1,000 to 1,500 would only fight hardening test C-1.