feat: add Undo action variant and actions-on-double-click

[clansty]

Summary

Closes #481

• Add undo as a new Action variant, usable in actions-on-right-click, actions-on-enter, actions-on-escape, and the new actions-on-double-click
• Add --actions-on-double-click as a configurable action list triggered by primary button double-click, falling back to actions-on-enter when not set
• Move right-click handling into update() where handle_undo() is accessible

Undo semantics

undo is a fallible action: if it succeeds, further actions in the list are skipped; if nothing to undo, processing continues to the next action. This enables patterns like:

--actions-on-right-click undo,exit

Which means: "undo one step; if nothing left to undo, exit (cancel)".

Double-click

Only fires when the active tool does not stop propagation — text tool word selection (n_pressed == 2 → RedrawAndStopPropagation) is unaffected.

[Copilot]

Pull request overview

Adds a new undo action variant and introduces a configurable double-click action list, expanding Satty’s configurable “action list” input triggers beyond Enter/Escape/Right-click.

Changes:

• Add Action::Undo and plumb it through configuration + CLI parsing.
• Add --actions-on-double-click (with fallback to actions-on-enter when unset).
• Move right-click handling into SketchBoard::update() and add process_actions() so undo can be executed from mouse-triggered action lists.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/sketch_board.rs	Adds process_actions() and handles right-click + double-click action triggering in update() so Undo can run with full board state.
src/configuration.rs	Adds Undo action variant and a new actions_on_double_click configuration field with merge/default/getter wiring.
cli/src/command_line.rs	Adds --actions-on-double-click CLI option and exposes undo in the CLI Action enum.
README.md	Documents undo as a possible action value and documents the new double-click actions option.

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[Copilot]

Action::Undo is only executed when action lists go through process_actions(). The Enter/Escape keypath still calls renderer.request_render(&actions) directly, so undo in actions_on_enter/actions_on_escape will be ignored (and e.g. undo,exit will always exit). Route those key-triggered action lists through process_actions() (or otherwise handle Undo before calling request_render) so the README/PR semantics hold for all supported triggers.