Tools: Frame tool (#2914) (#2915)

The Frame tool (K, its own toolbar slot after the Crop group) draws a
rectangular or elliptical frame; the options bar picks the shape, Shift
constrains to a square or circle and Alt draws from the centre. A frame
drawn over the selected pixel layer's content takes that layer in;
otherwise it is an empty frame.

Frames are built as Frame from Layers builds them: a group clipped by a
layer mask (rectangular or anti-aliased elliptical). The new engine
command layer.new.frame does it in one Session::edit, so one undo step.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

ATTRIBUTION.md

@@ -20,7 +20,7 @@ next to them.
 | `crates/ui-egui/src/i18n/uk.tsv` | Ukrainian UI translations | PhotoCraft contributors | Original translations of PhotoCraft's English labels | MIT OR Apache-2.0, [`LICENSE-translations.txt`](crates/ui-egui/src/i18n/LICENSE-translations.txt) |
 | `assets/fonts/Inter-Regular.ttf`, `Inter-Medium.ttf`, `Inter-SemiBold.ttf` | Inter 4.001 (UI font) | The Inter Project Authors (Rasmus Andersson) | <https://github.com/rsms/inter> | SIL OFL 1.1, [`assets/fonts/OFL-Inter.txt`](assets/fonts/OFL-Inter.txt) |
 | `assets/fonts/JetBrainsMono-Regular.ttf` | JetBrains Mono 2.305 (numeric font) | The JetBrains Mono Project Authors | <https://github.com/JetBrains/JetBrainsMono> | SIL OFL 1.1, [`assets/fonts/OFL-JetBrainsMono.txt`](assets/fonts/OFL-JetBrainsMono.txt) |
-| `assets/icons/*.svg` except `slice-knife.svg`, `artboard.svg`, `perspective-crop.svg`, `type-mask.svg`, `type-mask-vertical.svg`, `clip-below.svg`, `clip-release.svg`, `layer-mask.svg`, `adjustment-layer.svg`, `brush-replace.svg`, `eraser-background.svg`, `eraser-magic.svg`, `lasso-magnetic.svg`, `direct-select.svg`, `pen-add-anchor.svg`, `pen-delete-anchor.svg`, `pen-freeform.svg`, `pen-curvature.svg`, `bandage-sparkle.svg`, `painting-symmetry.svg`, `symmetry-*.svg` and `marquee-{row,column}.svg` (112 files; `star-filled.svg` is `star.svg` with a solid fill) | Lucide icons | Lucide Icons and Contributors | <https://github.com/lucide-icons/lucide> (`icons/<name>.svg`; `align-*` are now named `text-align-*` upstream) | ISC, [`assets/icons/LICENSE-lucide.txt`](assets/icons/LICENSE-lucide.txt) |
+| `assets/icons/*.svg` except `slice-knife.svg`, `artboard.svg`, `perspective-crop.svg`, `frame-placeholder.svg`, `type-mask.svg`, `type-mask-vertical.svg`, `clip-below.svg`, `clip-release.svg`, `layer-mask.svg`, `adjustment-layer.svg`, `brush-replace.svg`, `eraser-background.svg`, `eraser-magic.svg`, `lasso-magnetic.svg`, `direct-select.svg`, `pen-add-anchor.svg`, `pen-delete-anchor.svg`, `pen-freeform.svg`, `pen-curvature.svg`, `bandage-sparkle.svg`, `painting-symmetry.svg`, `symmetry-*.svg` and `marquee-{row,column}.svg` (112 files; `star-filled.svg` is `star.svg` with a solid fill) | Lucide icons | Lucide Icons and Contributors | <https://github.com/lucide-icons/lucide> (`icons/<name>.svg`; `align-*` are now named `text-align-*` upstream) | ISC, [`assets/icons/LICENSE-lucide.txt`](assets/icons/LICENSE-lucide.txt) |
 | `assets/icons/{check,chevron-down,chevron-right,chevron-up,chevrons-left,chevrons-right,circle,clock,compass,info,link,link-2,lock,minus,monitor,moon,move,navigation,plus,search,square,trash,triangle,type,x,zoom-in}.svg` | Lucide icons derived from Feather (subset of the row above) | Cole Bemis (Feather), Lucide Contributors | <https://github.com/feathericons/feather> via Lucide | MIT (Feather) and ISC (Lucide), [`assets/icons/LICENSE-lucide.txt`](assets/icons/LICENSE-lucide.txt) |
 | `assets/icons/slice-knife.svg` | Slice tool glyph | PhotoCraft contributors | Original work | MIT OR Apache-2.0 |
 | `assets/icons/artboard.svg` | Artboard tool glyph: a board with a tick at each end of its edges | PhotoCraft contributors | Original work | MIT OR Apache-2.0 |
@@ -29,6 +29,7 @@ next to them.
 | `assets/icons/marquee-row.svg`, `assets/icons/marquee-column.svg` | Single Row and Single Column Marquee tool glyphs: a dashed row (column) between two edge bars, drawn on Lucide's 24 px grid and stroke | PhotoCraft contributors | Original work | MIT OR Apache-2.0 |
 | `assets/icons/clip-below.svg`, `assets/icons/clip-release.svg` | Clipping-mask cursors (⌥ over the line between two layers): a hooked arrow onto the layer below; the same with a small "no" badge to release | PhotoCraft contributors | Original work | MIT OR Apache-2.0 |
 | `assets/icons/layer-mask.svg`, `assets/icons/adjustment-layer.svg` | Layers panel footer glyphs: Add a mask (a frame around a solid disc) and New Fill or Adjustment Layer (a circle, one half solid), drawn on Lucide's 24 px grid and stroke | PhotoCraft contributors | Original work | MIT OR Apache-2.0 |
+| `assets/icons/frame-placeholder.svg` | Frame tool glyph: an empty placeholder frame, crossed corner to corner | PhotoCraft contributors | Original work | MIT OR Apache-2.0 |
 | `assets/icons/eraser-background.svg`, `assets/icons/eraser-magic.svg` | Background Eraser and Magic Eraser tool glyphs: Lucide `eraser` plus added marks | Lucide Icons and Contributors; marks by PhotoCraft contributors | Derived from <https://github.com/lucide-icons/lucide> `icons/eraser.svg` | ISC, [`assets/icons/LICENSE-lucide.txt`](assets/icons/LICENSE-lucide.txt) |
 | `assets/icons/brush-replace.svg` | Color Replacement tool glyph: Lucide `brush` plus two swap arrows | Lucide Icons and Contributors; arrows by PhotoCraft contributors | Derived from <https://github.com/lucide-icons/lucide> `icons/brush.svg` | ISC, [`assets/icons/LICENSE-lucide.txt`](assets/icons/LICENSE-lucide.txt) |
 | `assets/icons/bandage-sparkle.svg` | Remove tool glyph: Lucide `bandage` moved down, plus a sparkle mark | Lucide Icons and Contributors; mark by PhotoCraft contributors | Derived from <https://github.com/lucide-icons/lucide> `icons/bandage.svg` | ISC, [`assets/icons/LICENSE-lucide.txt`](assets/icons/LICENSE-lucide.txt) |