perf(media): tell the media pool which EXR footage effects read (#474)

The pool learned that an effect reads a footage item's channels only at the
first aux() call, so the first frame was decoded twice (picture, then in
full), and the hint was never cleared after the effect was removed.

The session now gives the footage source the whole set of files read by
enabled 3D Channel effects (FootageSource::set_aux_footage) with every
project change (Session::bump: edits, undo, redo, history jumps, rollbacks,
project open), sending it only when it changed. The pool keeps that set and,
as a safety net, what aux() learns; a new set drops what was learned.

Refs #474

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

docs/file-format-parity.md

@@ -41,7 +41,7 @@ trips and renders, checked by tests, not against After Effects), **partial** (na
 | PNG | R/W | yes | yes (sequence) | good |
 | JPEG | R/W | yes | yes (sequence) | good |
 | TIFF | R/W | yes | yes (sequence) | good |
-| OpenEXR (multi-layer, Cryptomatte) | R/W | yes | yes (32-bit sequence) | good; HTJ2K compression fails (#482); a multi-layer frame decodes once for its picture and EXtractoR / Cryptomatte, except the first frame an effect reads and files whose picture the `image` crate still decodes (#474); footage alone keeps only its picture's channels, though a single-part file still decompresses every channel's pixels; decoded channels are held as f32 |
+| OpenEXR (multi-layer, Cryptomatte) | R/W | yes | yes (32-bit sequence) | good; HTJ2K compression fails (#482); a multi-layer frame decodes once for its picture and EXtractoR / Cryptomatte from the first frame on, except files whose picture the `image` crate still decodes (#474); footage alone keeps only its picture's channels, though a single-part file still decompresses every channel's pixels; decoded channels are held as f32 |
 | Photoshop PSD / PSB | R (footage, composition) / W (layers, sequence) | yes (layers, groups, masks, styles, smart objects) | Save Frame As ▸ Photoshop Layers; no Photoshop sequence output | good, from Adobe's published spec |
 | Illustrator `.ai`, PDF, EPS | R | yes (vector, Continuously Rasterize, Create Shapes; Composition and Composition - Retain Layer Sizes, #1536) | — | partial: no JPX / JBIG2 images, no Illustrator procset EPS |
 | SVG | R | yes | — | good; SVG markup on the clipboard pastes as a shape layer (26.3, #738); Illustrator's own clipboard data (PDF/AI) doesn't |

docs/footage.md

@@ -122,10 +122,12 @@ vectors, positions and IDs are never the picture. EXtractoR reads every other la
 
 Footage on its own keeps only the channels of its picture (a single-part file still
 decompresses all of its pixels to read them). Once an effect reads an item's channels
-(EXtractoR, Cryptomatte, the 3D Channel effects), each later frame is decoded once for the
-picture and the effects together, and kept for a while: about 1 GB of decoded channels (512 MB
-in the browser), a soft bound, as the newest file and files still in use stay whatever their
-size. Several frames asking for the same file at once wait for one decode.
+(EXtractoR, Cryptomatte, the 3D Channel effects), each frame, the first included, is decoded
+once for the picture and the effects together, and kept for a while: about 1 GB of decoded
+channels (512 MB in the browser), a soft bound, as the newest file and files still in use
+stay whatever their size. Several frames asking for the same file at once wait for one
+decode. Once no enabled effect reads the item any more, its frames go back to the picture's
+channels alone.
 
 ## Replacing and reloading