Load source images in the background with a progress window

Decoding a batch of photographs took seconds on the main thread, so the
window froze with no sign of what was happening. loadImages now takes an
optional LoadProgress (per-file callback plus a cancel flag) and hands files
out from a shared cursor, so one large file cannot stall the queue behind it.

The GUI decodes off the main thread and shows a progress window with the
count, the current file and a Cancel button; it only appears if the batch is
still running after 250 ms. Headless mode keeps a "reading n/total" line on
stderr when it is attached to a terminal.

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

README.md

@@ -37,6 +37,13 @@ build/effector -o out.png -s 1920x1080 \
 Any option can also be passed when launching the interface; it becomes the
 starting state (images, canvas size, effect settings, seed).
 
+Reading a batch of large photographs takes seconds, so it never blocks the
+interface: decoding runs in the background and a progress window shows how many
+files are done, which one is being read, and a Cancel button that stops the
+batch. Small batches finish before the window would be drawn, so it only appears
+if the decode is still going after 250 ms. On a terminal the headless mode keeps
+a single `reading n/total` line updated on stderr; redirected output stays clean.
+
 ## Effects
 
 ### scanlines
@@ -113,3 +120,7 @@ are decoded in parallel too, which is what dominates a headless run.
 A 7680×4320 canvas at `--strip-height 2` from three multi-megapixel JPEGs renders
 in ~35 ms, against ~73 ms for the Python prototype; the interface reports the
 render time of the current preview in its status line.
+
+Decoding threads take the next file from a shared cursor rather than a fixed
+slice of the list, so one very large file cannot stall the files queued behind
+it and progress advances at a steady rate.