Narrow selection on cross-comparison mark_as_reference

When an external client marks reference entries across multiple
comparisons in sequence (e.g. an MCP agent calling library.set_reference
once per file in a target directory), each call landed in a different
comparison and inserted into the selection unconditionally. The result
violated the Group-by-Name invariant that the selection lives in a
single comparison: the viewport ended up diffing unrelated images, and
the GUI Group-by-Name list could no longer reason about an active
group.

When the new reference targets a different comparison than the current
coherent single-comparison selection, narrow the selection to the new
comparison's members first -- the same behaviour as click_in_group()'s
group-switch branch. Heterogeneous (flat-mode) and empty selections
still fall through to the legacy insert path so cross-comparison flat
selections continue to work.

Per-comparison references recorded along the way are unaffected:
revisiting an earlier comparison still restores its pinned reference
via apply_comparison_reference.

Define IDIFF_TEST_SAR_VIDEO_PATH when ffmpeg CLI is absent

The else-branch only defined IDIFF_TEST_VIDEO_PATH, leaving
IDIFF_TEST_SAR_VIDEO_PATH undeclared on Windows CI where ffmpeg
is not installed.

Strip path separators when computing group keys

The image-list "Group by Name" view buckets entries by the stem of
ImageEntry::filename. The comparison-config flow can override that
field with a path-style display label such as "seedvr2_1x/role1.png"
and "4k/role1.png", which previously hashed to two different stems
("seedvr2_1x/role1" and "4k/role1") and turned every entry into its
own singleton group. Clicks then never matched the active group, so
the panel ended up with selected entries scattered across several
visual groups instead of switching atomically.

Strip the leading directory from the input before splitting off the
extension, treating both '/' and '\\' as separators. Existing call
sites that already pass a bare basename are unaffected.

Strip path separators when computing group keys

The image-list "Group by Name" view buckets entries by the stem of
ImageEntry::filename. The comparison-config flow can override that
field with a path-style display label such as "seedvr2_1x/role1.png"
and "4k/role1.png", which previously hashed to two different stems
("seedvr2_1x/role1" and "4k/role1") and turned every entry into its
own singleton group. Clicks then never matched the active group, so
the panel ended up with selected entries scattered across several
visual groups instead of switching atomically.

Strip the leading directory from the input before splitting off the
extension, treating both '/' and '\\' as separators. Existing call
sites that already pass a bare basename are unaffected.

Fix YUV parameter dialog, add FFmpeg fallback, and show load errors

YUV dialog overhaul: pixel-format Custom field now activates on click;
colour metadata is no longer guessed from filenames and unknown values
are dropped on load. All dialog settings persist across sessions.

FFmpeg fallback: files whose extension is not in the image loader
whitelist are now routed to FFmpeg (covers y4m and other containers).

Load failure feedback: a modal dialog now appears when an image fails
to load or reload, replacing the easy-to-miss status-bar-only message.

Group by Name: new filename-based grouping with visual indicators,
Shift+click range select, and Select Group context menu.

Document 0.2.2 release in changelog

Release 0.2.1

Bump version to 0.2.0 and update CHANGELOG

Revert "Add code review document for origin/master..master"

This reverts commit a218caa.

Add CHANGELOG entry for v0.1.1