Reload a numbered still as its own file (#2074)

Reload Footage probed every item the way File ▸ Import does by default,
so a still such as Photo_2.jpg with Photo_1.jpg beside it was grouped
into a run and measured from the run's first file. The still took
Photo_1's size. Sequence items still probe as a run; everything else
reads its one file.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/footage.md

@@ -129,7 +129,8 @@ including files overwritten at the same path and byte size.
   File ▸ Open tries each missing file at the same place under the project's new folder, without
   marking the project modified.
 - **File ▸ Reload Footage** (Ctrl+Alt+L) reads the files again. A sequence picks up frames
-  added to its folder (a render still in progress) and keeps its interpretation.
+  added to its folder (a render still in progress) and keeps its interpretation; a still is
+  read as its one file, even when numbered files sit next to it.
 
 **File ▸ Dependencies ▸ Relink Missing Footage…** searches a chosen folder and its subfolders
 for missing source files on the desktop. It relinks unique exact filenames whose decoded type,