Brush and Pencil: hold ~ to erase, and Clear mode (#3439) (#3465)

* Brush and Pencil: hold ~ to erase

Fixes #3439. Holding ~ (the backtick key) with the Brush or Pencil makes
the strokes started while it is held erase, with that tool's own brush,
as in Photoshop; releasing it paints again and the tool never changes.
The key is a Tools › Temporary binding (tools.temporary.erase), so it
can be rebound or removed in Keyboard Shortcuts.

* Brush and Pencil: Clear painting mode, used by ~

Photoshop's Clear joins the Brush's and Pencil's Mode menus, after
Dissolve: chosen there, every stroke erases with that tool's brush until
another mode is picked. Holding ~ now puts the stroke in Clear mode too,
and the Mode menu shows Clear while the key is held. History names the
tool rather than the Eraser. Agents get it as "mode": "clear" on
paint.stroke and paint.pencil.

* Keep the Smoothing doc comment on smoothing_field

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

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -188,7 +188,7 @@ the same commands work in the app, the CLI and headless MCP. Paths are dotted ca
 | `prefs.set` | `{"path":"cursors.painting","value":"precise"}` or `{"values":{"unitsAndRulers.rulers":"cm","guidesGridAndSlices.gridColor":"#ff8800"}}`. Values are validated (choices, ranges, `#rrggbb` colours, shortcut syntax); a batch applies all or nothing. A section path takes an object and merges it key by key |
 | `prefs.reset` | `{"path":"performance"}` (a section or key); no path resets everything. An individual `shortcuts.<id>` or `menus.colors.<id>` reset removes the override and returns `null`, including when it is already absent |
 | `edit.preferences.<section>` | the section's values; in the app (no params) it opens the Preferences dialog on that section |
-| `edit.keyboardShortcuts` | `{"set":{"edit.fill":"Cmd+Shift+F"},"reset":true\|["id",…],"removeConflicts":true,"filter":"blur","list":false,"importKys":"<.kys XML>"}`: returns overrides, matching commands and conflicts. A shortcut moved to another command is removed from its old owner unless `removeConflicts` is false. `""` removes a shortcut, `null` restores the default. The held temporary tools are bindable too: `tools.temporary.hand` (Space; also repositions a selection being drawn), `tools.temporary.zoomIn` (Cmd+Space), `tools.temporary.zoomOut` (Alt+Space; Alt added to a held Zoom In, Cmd+Alt+Space, zooms out too); they list with `"hold": true`. ⌘ (Ctrl off the Mac) held alone is the Move tool with the painting, retouching and other non-selection tools, as in Photoshop; it is a modifier, not a binding. `importKys` applies a Photoshop `.kys` set (the file's XML text, at most 4 MiB): rows are matched to commands by menu label, a key equal to the default restores it, keys also given in `set` win, tool keys are not imported, and the result gains `import` (`name`, `imported`, `set`, `unknown`, `unreadable`, `alternates`, `toolKeys`). In the app it opens Keyboard Shortcuts and Menus, whose Import Shortcuts… button loads a `.kys` set into the dialog (opening or dropping a `.kys`, or Photoshop's live `Keyboard Shortcuts.psp`, does the same). On a desktop with Photoshop installed, the first launch that has no shortcut overrides yet asks whether to import the newest install's live set (Import / Don't Import; `ui.inspect` shows it as `kysOffer`); the answer is recorded in the preferences as `dialogs.photoshopShortcuts`, so it is asked once |
+| `edit.keyboardShortcuts` | `{"set":{"edit.fill":"Cmd+Shift+F"},"reset":true\|["id",…],"removeConflicts":true,"filter":"blur","list":false,"importKys":"<.kys XML>"}`: returns overrides, matching commands and conflicts. A shortcut moved to another command is removed from its old owner unless `removeConflicts` is false. `""` removes a shortcut, `null` restores the default. The held temporary tools are bindable too: `tools.temporary.hand` (Space; also repositions a selection being drawn), `tools.temporary.zoomIn` (Cmd+Space), `tools.temporary.zoomOut` (Alt+Space; Alt added to a held Zoom In, Cmd+Alt+Space, zooms out too), `tools.temporary.erase` (`` ` ``; with the Brush or Pencil, a stroke started while it is held is in Clear mode, `paint.stroke`/`paint.pencil` `"mode": "clear"`, erasing with that tool's brush); they list with `"hold": true`. ⌘ (Ctrl off the Mac) held alone is the Move tool with the painting, retouching and other non-selection tools, as in Photoshop; it is a modifier, not a binding. `importKys` applies a Photoshop `.kys` set (the file's XML text, at most 4 MiB): rows are matched to commands by menu label, a key equal to the default restores it, keys also given in `set` win, tool keys are not imported, and the result gains `import` (`name`, `imported`, `set`, `unknown`, `unreadable`, `alternates`, `toolKeys`). In the app it opens Keyboard Shortcuts and Menus, whose Import Shortcuts… button loads a `.kys` set into the dialog (opening or dropping a `.kys`, or Photoshop's live `Keyboard Shortcuts.psp`, does the same). On a desktop with Photoshop installed, the first launch that has no shortcut overrides yet asks whether to import the newest install's live set (Import / Don't Import; `ui.inspect` shows it as `kysOffer`); the answer is recorded in the preferences as `dialogs.photoshopShortcuts`, so it is asked once |
 | `edit.menus` / `edit.toolbar` | `{"hide":["edit.fade"],"show":[…],"color":{"edit.fill":"red"},"reset":false}` / `{"hidden":["Sponge"],"order":[…]}` |
 | `edit.colorSettings` | `{"workingRgb":"display-p3","workingCmyk":"coated-cmyk","workingGray":"sgray","policyRgb":"preserve\|convert\|off",…,"askOnMismatch":true,"intent":"perceptual","bpc":true}`; honoured when opening files (`file.openAs`, the app's File › Open) and by Image › Mode and "working" profile specs. `color.profileMismatch {"action":"preserve\|convert\|discard\|assignWorking"}` answers the mismatch prompt |