PR #18435 — live isolated Linux web/server browser proof

Same local CSV fixture (37 bytes): name,result\npreview,proof-18435-75d9\n
Baseline: remove only await download.delete().catch(constVoid) from the actual server source.
A fresh default-profile Chromium context retains one 37-byte original in its Playwright artifact directory after saving. T3 saved copy remains served over its real authenticated HTTP download endpoint: 200, exact CSV bytes.
Fixed: restore the original source exactly and restart the isolated server.
A fresh default-profile Chromium context has zero original artifacts after saving. T3 saved copy is 37 bytes and remains served by the same endpoint: 200, exact CSV bytes.
Both filesystem and HTTP checks happened while each context remained open. The Chromium process was traced as a descendant of the isolated worktree server; its user-data-dir was the worktree's default profile. Distinct fixture bytes matched the baseline original and both served copies.

Final focused suite: 43→45 passed. Omitting cleanup fails its regression; moving deletion before settlement fails navigation ordering. Removed an inaccurate browser-failed-download mock: real Playwright may reject deletion on that path. No new claim about canceled partial files.
Transient Save toast click timed out; verified the actual download endpoint directly from the browser. No UI change in this PR.
Not verified: macOS, Windows, iOS, Android, Electron desktop.