Mark a burst in the grid, and let it be folded away

The counterpart to the grouping: where the signatures come from, and how a
group reaches a cell.

Signatures are computed from the 256px thumbnails dr-thumbs already holds --
vastly more resolution than a 9x8 reduction can use -- so a library that has
been browsed, or that has synced somebody else's shards, has already paid for
them and no RAW is decoded for this. The consequence is stated rather than
hidden: an image with no thumbnail gets no signature and never joins a burst.
That is self-correcting, and it is why the pass runs when the thumbnail sweep
finishes rather than on a timer. Nothing happens at import and nothing happens
at query time.

The mark is drawn as a child of the cell's TouchArea, for the same reason the
star strip is: a click on it must not also reach `cell-clicked` and throw the
user into develop, and children are hit-tested before the element they sit in.
It is never hidden on hover the way the stars are -- a collapsed burst stands
in for frames that are not on screen, and something has to say so whether or
not a pointer is nearby.

Folding changes what the grid's *query* returns rather than what its cells
draw, because the grid is a window over an ordered query and the frames a fold
hides are mostly not loaded. So the predicate joins VISIBLE in every query
that lists or counts cells -- the window, the header's count, the run a
shift-click resolves, and the ordinal a scrub lands on -- under the discipline
VISIBLE's own comment sets out: present in four places of five is worse than
absent, because the counts disagree with the cells and neither looks wrong on
its own. There is a test for exactly that.

`the_window_read_walks_the_ordering_index` now includes the burst clause. It
asserts on the query plan while holding its own copy of the query, so left
alone it would have gone on reporting green against a query the grid no longer
runs. If the clause costs `images_grid_order` and puts the sort back, that
fails here rather than becoming jitter someone measures in six months.

The pass keeps its own drain timer in a thread-local instead of taking fields
on the library controller, so everything the feature needs to run lives in one
file and the screen that starts it holds nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/lib.rs

@@ -20,6 +20,7 @@
 //! in `ui/` names an operation or knows a shader exists (FR-DEV-3a).
 
 mod activity;
+mod bursts;
 mod collections_ui;
 mod derived_sync;
 mod develop;