Export a whole selection, on a thread that is not the interface's

The export button rendered, resampled and encoded a 24 MP frame on the UI
thread and the window was dead for all of it. That was written down as a known
compromise, on the grounds that a batch is what makes the wait intolerable
rather than merely noticeable. This is the batch, so the compromise comes due.

The grid's selection now exports (FR-EXP-7). A worker thread takes a clone of
the `GpuContext` — an `Arc` pair over a device and a queue — and opens each
photograph for itself: fetch, sidecar, decode, demosaic, render at full size,
resample, sharpen, encode, write. Nothing of that touches the interface, which
keeps drawing throughout, and the progress goes where every other background
job's does: one row in the activity register, with a count and a bar.

Why the worker does not borrow the session it could have had. A
`DevelopSession` owns the `AdjustPass` the canvas renders from, so handing it
to a worker would stop the develop view drawing for the length of the batch —
the same freeze, moved. Opening a session per image instead costs a
`Demosaicer` and an `AdjustPass` each time round, and the pipeline cache is
per-pass so the composed shader is recompiled per image rather than once for
the run. Against a full-resolution decode, render and encode that is a few
percent, and it keeps this file out of the pipeline `develop` owns. A reusable
export pass is the obvious next economy if a profile ever says so.

The open image is the exception, and it is why the develop button is not simply
a one-image batch. Its edit lives in the interface's session and may not have
reached a sidecar yet, so a worker that re-opened the file would export the
saved version rather than the one on screen. That frame is therefore rendered
by the caller and handed over as `Source::Rendered`; everything after the
render — the Lanczos reduction, the encode, the write, which is the larger half
of the wait and all of its variance — still leaves the UI thread. So the
develop export is no longer synchronous, but it is not fully off-thread either,
and the doc comment says so rather than claiming otherwise.

Cancellation (NFR-ARCH-3) is an `AtomicBool` read between stages, and the
export button becomes the cancel button while a run is live — a batch that
could only be stopped by not touching the selection would be a trap. Waits on
another worker use `recv_timeout` rather than `recv`, so a cancelled batch
sitting on a forty-megabyte download gives up within 100 ms instead of when the
transfer finishes. The honest bound is worse than that: a frame already in
render has no interior stopping point, so the worst case is one image. Closing
that needs the render itself to become interruptible, which is NFR-ARCH-2's
scheduler and not a finer poll here.

Failures are per image and typed (NFR-ARCH-4). One unreadable body, one folder
that cannot be written, one server that went away — each is a message on the
channel, a line in the log, and a count in the summary, and the batch carries
on. A run with any failure keeps its row until it is cleared, because that is
the row somebody came to the list to find; a cancelled run does not, because
they asked for it.

Two collisions that look alike and are not. `CollisionPolicy` is the user's
answer to "a file of this name was already there", and Overwrite is a fine
answer to that. It is not an answer to "the frame I exported four seconds ago
was also called this" — two folders in a library each holding an IMG_0001 is
ordinary — so a name the run has already issued is always stepped past whatever
the policy says about the folder. Both halves are held by tests.

Supporting changes, each smaller than it sounds. `open_session` comes out of
`load_bytes` so the worker shares the JPEG-versus-RAW routing rather than
carrying a copy that would drift; the half that builds a `slint::Image` stays
behind, where it belongs. `LibraryController::selected_image_paths` answers
from the catalog rather than from the loaded window, because selection is by id
and survives a scrub — a selection made before scrolling routinely names
photographs no row holds. `cache_context_for` takes an id for the same reason,
so a batch reads the originals cache instead of re-downloading three hundred
files. `format_date` is shared so `{date}` and the timeline agree about what
day a photograph was taken.

Left undone, deliberately: the batch is sequential, where FR-EXP-7 asks for all
available cores. Four full-resolution frames in flight is tens of megabytes
each and a straightforward way to exhaust a tablet, and the GPU is shared with
the interface in any case. Also undone: exporting with a chosen preset rather
than the current export settings — that is FR-EXP-5's machinery, which does not
exist yet.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/dr-ui/src/activity.rs

@@ -80,6 +80,9 @@ pub enum Kind {
     Sync,
     /// Moving files to the trash, restoring them, or emptying it.
     Trash,
+    /// TRACES: FR-EXP-7
+    /// Rendering and writing finished files.
+    Export,
 }
 
 impl Kind {
@@ -94,7 +97,14 @@ impl Kind {
             | Kind::Download
             | Kind::Upload
             | Kind::Sync
-            | Kind::Trash => true,
+            | Kind::Trash
+            // A batch export fetches every original it does not already have
+            // cached, which on a selection of three hundred RAW files is the
+            // largest transfer this application ever starts. It moves nothing
+            // at all when the whole selection is already on disk, but the coarse
+            // answer has to be the cautious one: the question this flag answers
+            // is asked by somebody on a metered connection.
+            | Kind::Export => true,
             // Reads headers over the network today, but it is bounded by the
             // catalog rather than by anything the user asked to move, and it
             // stores nothing. Calling it a transfer would put an hours-long