Image sequences: a gap in the numbering holds the previous frame, as After Effects (#3137)

* Image sequences: a gap in the numbering holds the previous frame, as After Effects

Split out of #1876 (originally 7850cbf4), because it reverses the colour bars
documented in #297 and needs its own discussion.

After Effects 2026 shows the last existing file across a gap in a sequence's
numbering (files 1, 2, 4, 5 show frame 2 at frame 3; read back from After
Effects). EffectCraft drew colour bars there. The media pool now holds the
previous file (Footage::sequence_file_held); colour bars remain only where no
file precedes the frame. Missing-frame reports on import are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Image sequences: hold the first file of the preceding number across a gap; fix the import warning

A gap now holds the file sequence_file picks for the preceding number, so
files sharing a number do not switch at the gap. The Report Missing Frames
warning says the previous frame is held instead of "shown as placeholders".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Image sequences: Interpret Footage says a gap holds the previous frame

The Interpret Footage note for a numbered sequence with gaps still said the
missing frames are shown as colour bars; the media pool holds the previous
frame there now. The note, its comment and two test doc comments say so,
and a forms test checks the note's wording.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: FredApps <153173854+FredApps@users.noreply.github.com>

docs/agents.md

@@ -302,8 +302,8 @@ build real projects through these interfaces; they are worked examples of everyt
   file's whole numbered run as one item (several picked frames: that range; a folder: each run in
   it, also without `sequence`). Without `"sequence": true` a picked or dropped numbered file imports
   as one still, as in After Effects; `"alphabetical": true`: Force Alphabetical Order, the files
-  play one after another) at `frameRate` or 30 fps (Settings ▸ Import). Missing frames show
-  colour bars, as in After Effects. `file.interpretFootage {"items":[id], "frameRate":12,
+  play one after another) at `frameRate` or 30 fps (Settings ▸ Import). A gap in the numbering holds
+  the previous frame, as in After Effects. `file.interpretFootage {"items":[id], "frameRate":12,
   "startTimecode":1001}` conforms them and sets Override Start (the item and layers that ran to its end get the new
   length). See [footage.md](footage.md).
 * **Colour of footage**: `file.interpretFootage {"items":[id], "preserveRgb":true}` is Interpret