fix(files): mark folders the scan limit stopped short of

The fair rounds give a directory its node before reading it, so every folder
still queued when the 5,000-node budget ends sat in the tree with empty or
partial children and read as an empty folder. Keep the node on the tree cursor
and set an additive incomplete: true on every cursor left in the queue, then
draw one dim, inert row under such a folder while it is expanded. The envelope
check ignores keys it does not know, so old and new clients stay compatible.

Stop each round at the top once the budget is spent, before the shift: the
rest stay queued unread, and flagged, rather than read and sorted for nothing.

Say what the walk is: a per-directory round-robin (breadth-first), fair per
directory and not per subtree, with search results now shallow-first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changeset/quiet-folders-share.md

@@ -2,4 +2,4 @@
 'aicodeman': patch
 ---
 
-Share File Viewer scan budgets across sibling directories so large early folders do not hide later results.
+Files panel: one large folder no longer uses up the 5,000-entry scan limit and hides the folders after it. The tree and its search now walk folders in a per-directory round-robin (breadth-first), so search results come back shallow-first rather than depth-first. This is fair per directory, not per subtree: a folder made of thousands of small subfolders can still use most of the limit one level down. A folder the limit stopped short of no longer looks empty: `GET /api/sessions/:id/files` marks it `incomplete: true` (additive) and, when expanded, it shows a dim "Not fully scanned (file limit reached)" row.