Fix: Large folders with many PDFs load slowly

[MemerGamer]

Findings

• Root cause of the "never finishes loading" Downloads: Files groups Downloads by date created by default, and Uno's grouped CollectionView renders no rows. The headless repro (301 items: PDF/PNG/ZIP/DOCX/XLSX/JPG) showed an empty list for 45s+ even though the status bar counted the items; any ungrouped folder of the same size listed within ~2s. The default is now skipped on Linux (LINUX-TODO(listing) left for grouped rendering generally; manually chosen grouping is untouched and still affected).
• Thumbnails were already lazy (realized rows only), bounded (service gate 1-4), and served from the shared freedesktop cache first. Hardened: the cache lookup did synchronous file reads on the UI thread (now Task.Run), the item-load cancellation token is forwarded so scrolled-away or navigated-away requests stop waiting on the gate, and settle-pass loads are capped at 4 parallel.
• Per-zip inspection and extended properties are not done during listing. The CloseWatcher line was once per navigation (not a loop in the sandbox); it is now Debug level. I could not reproduce a watcher open/close loop headless, so real-desktop churn (e.g. an active download or inotify overflow triggering full refresh) is unconfirmed.

Measured (headless, Xvfb, nice 19, software rendering)

• Before: Downloads blank >45s (Details and Grid). After: rows visible in ~2-3s; generic icons first, PDF/JPG thumbnails fill in for visible rows over ~5-14s with 4 concurrent generations.

Tests: Files.Platform.Tests 864 passed, 7 skipped.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LeKXJc3DnK3PzAqwgA72NY