Fix #485: the mouse wheel steps the focused numeric field

[bflatastic]

The wheel did nothing over numeric fields; as in Illustrator, once a numeric field has the focus, turning the wheel over it now steps its value like Up/Down.

• One code path (widgets::step_value, formerly step_with_arrows from Panel number fields: Up/Down arrows step the value (Shift ×10, Ctrl ÷10) #357): arrows and wheel step num_field, plain_field/mixed_field and the spin fields built on them (panels, Control bar, dialogs, the Font Size / Leading combo boxes from Fix #458: Font Size and Leading combo boxes in the Character and Properties panels and the Control bar #459), and now opt_field (Stroke dash/gap) too. A notch is one step, Shift ten, Ctrl/Cmd a tenth; trackpad and fine-wheel points add up to notches. The new value is applied at once and selected.
• No hijacking: only a focused field under the pointer takes the wheel, and the panel under it then stays put; an unfocused field, or the pointer anywhere else (panel, canvas), scrolls/zooms as before.
• Dialogs and Preferences: their egui DragValues become the same recessed number fields (widgets::range_field: clamped to their range and rounded to their precision), so they type, step and scroll like the rest (Effect gamma, the generic command/effect/plug-in parameters, Layer and Layers Panel Options, PNG/JPEG/GIF and Save for Web counts, export options, numeric preferences).
• Docs (docs/control-protocol.md ui.wheel), ROADMAP (Last updated, the number-field bullet). No new UI strings.

Illustrator reference: values in panels and dialogs step with the arrow keys once the field is clicked (Set precise property values); the wheel steps a field the same way after it is clicked (black-box behaviour; helpx doesn't mention the wheel).

Checks

• Rebased on d9fc3b6505bb185c7ecce05ab4b60b86da12822f (origin/main).
• cargo fmt --all --check; cargo clippy -p vectorcraft-ui-egui --all-targets -- -D warnings: clean.
• cargo test -p vectorcraft-ui-egui: 714 passed. New: the_wheel_steps_a_focused_field_under_the_pointer (lines, Shift/Ctrl, points carried over, unfocused and pointer-elsewhere do nothing, plain fields) and the_wheel_scrolls_the_panel_unless_over_the_focused_field (a ScrollArea scrolls over an unfocused field, stays put over the focused one). Both fail on main.
• cargo xtask layers, cargo xtask brands, cargo xtask wasm: OK; cargo check -p vectorcraft-ui-egui --target x86_64-unknown-linux-gnu / aarch64-apple-darwin: OK.
• Live (desktop app, control port ui.wheel/ui.click/ui.screenshot): Control bar W: wheel over it unfocused → unchanged; clicked, +1 notch → 35.2778 → 36.278 mm, Shift +2 → 56.278, Ctrl −1 → 56.178 (value selected); the wheel over the canvas with the field focused leaves the value alone. Preferences › Smart Guides › Snapping Tolerance (a former drag value): clicked, +2 notches → 4 pt → 6 pt (dialog cancelled).

🤖 Generated with Claude Code

[bflatastic]

Full cargo xtask ci passed on the integrated stack; merging.