Canvas: double-clicking type with a selection tool puts the caret there

[theironchef]

Fixes #283.

Problem: the Selection tool's double-click on type returns only SwitchTool("type") (crates/tools/src/select.rs), so the Type tool started idle, with nothing being edited and no caret. Typing right after went to the shortcut handler instead of the text. X, for one, swapped the selected text's fill and stroke and turned it into outlined type.

Change: in handle_input (crates/ui-egui/src/canvas.rs), when a double-click switches the tool to Type, the canvas passes the double-click's position on to the Type tool as a click (Down + Up). A click on existing type already starts editing it with the caret at the clicked character (edit_at), so typing edits the text straight away, as in the reference app. Other double-clicks are unchanged: group isolation, exiting isolation, and the Type tool's own word selection.

Tests

• New canvas::tests::double_clicking_type_with_the_selection_tool_puts_the_caret_there: a headless double-click in the middle of "Hello world" with the Selection tool leaves the Type tool editing that text, with the caret strictly inside it. Typing X then inserts at the caret.
• With the new block disabled, the test fails (nothing is being edited).
• cargo test -p vectorcraft-ui-egui: 557 passed. cargo test -p vectorcraft-engine --lib persp_text: 12 passed (perspective type's double-click path).
• cargo clippy -p vectorcraft-ui-egui --all-targets -- -D warnings (Rust 1.99): clean.
• cargo fmt --all -- --check: clean.
• Checked by hand in a release build on Windows with a real double-click on "Hello world" and real typing: the Type tool was editing it with the caret at byte 6, where it was clicked, and typing X gave "Hello Xworld" in one "Typing" step. The fill and stroke were unchanged.

I didn't run the full cargo xtask ci (wasm step) here.

🤖 Generated with Claude Code

[bflatastic]

Thanks @theironchef, merged! I rebased it on main (both canvas tests kept next to the others) and added an "Editing type" line to the ROADMAP. Live check: a double-click on "Hello world" with the Selection tool switched to the Type tool with the caret at 6, and typing X gave "Hello Xworld" instead of swapping fill and stroke.