Links: a link check whose worker ends without a result no longer stops later checks

When the link-watch worker unwound before storing its stamps,
poll_link_scan returned None on every frame and left link_scan set. The
app then asked for a repaint every 100 ms and start_link_scan returned
early, so the app did not look for changed linked files in the
background again for the rest of the session.

poll_link_scan now clears a look that has ended without stamps. The new
ended_without_stamps helper holds the slot's lock across its read of the
stamps and its read of the ended flag, so a look whose worker stores its
stamps during the check is applied. The app goes back to its two-second
wake, and the next look starts on schedule. A look still running and a
look that stored its stamps behave as before.

docs/target-app-parity.md

@@ -1300,6 +1300,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - On Windows, a link on a drive mapped to a network share other than the document's (`Y:\…`) is missing, with the reason that the path is on another computer, as a link stored with that share's own path is; links on this computer's drives and on the document's share are followed.
   - In the Links panel, Shift-click selects a range of links and Ctrl/Cmd-click adds or removes one.
   - Linked files another app changes while their document is open follow Preferences › File Handling › Update Links, as they do when it opens: Automatically reads them again (an Update Links undo step), Ask When Modified offers to, Manually leaves them modified in the Links panel. The desktop app compares the active document’s linked files by size and modification time every two seconds and when its window comes back to the front, on a worker thread; a file still being written is tried again (`links.updateChanged`).
+  - A look at the linked files whose worker thread ends without a result, such as after a panic, no longer stops later looks: the app clears it, stops asking for a frame every 100 ms, and the next look starts on schedule (`crates/engine/src/tests/linkwatch.rs`, `crates/ui-egui/src/tests/place.rs`).
   - Import and paste, 2026-10-13: a multi-artboard `.ai` with notes beside its artboards opens from its editing data, a page's art past its artboard no longer lying over the page before it (#1528); a `.ai` gradient whose stops are listed upwards keeps each midpoint on the stop that starts its part of the ramp (#1530); SVG text on a path follows basic shapes (circle, ellipse, rect, line, polyline, polygon) as their SVG 2 equivalent paths (#1531, fixes #1527); PDFs whose alpha soft masks are drawn as images import with the mask instead of empty art (#1551); EPS and PostScript type set in Type 42 (TrueType) fonts imports at its size instead of 1000 times too large (#1553); PDF and EPS import keep a clipping path whose bounding box covers the page but whose shape doesn't (#1544, fixes #1542). Opening or placing a file with an image too large to decode says so, and the Links panel shows it on the row (#1540, part of #1479). Paste takes a PDF or EPS another app copied as editable vector art: from the macOS pasteboard (with SVG), EPS on Windows beside PDF, and `.eps` files copied in a file manager, with the new command `clipboard.importEps` (#1538, fixes #1499).
   - A `.ai` or `.ait` file copied in a file manager pastes as its art: one that holds a PDF pastes the objects of its editing data, and one saved as PostScript pastes as a copied `.eps` file does.
   - A copied `.ai` or `.ait` file that holds a PDF pastes the objects of its editing data as File › Open reads them: every layer's objects go into the active layer in their stacking order, with their names, hidden objects and type objects, instead of the PDF page's art with its type as outlines. When the editing data can't be read, the file pastes its page as before (`clipboard.importPdf {editingData}`). The objects of a hidden layer come in hidden, from a pasted EPS too.