Closes MINOR #9 theme axis dropdown gap. Chip-click semantics
change from "cycle to next value" to "toggle dropdown for direct
pick", matching TS's behavior where users see all axis values in
a menu and can pick any one.
Surface change:
- `Document.ui.axis_dropdown_open: Option<String>` (axis name)
- `VariablesPanelHit::AxisDropdownItem { axis, value }` new hit kind
- `VariablesPanel` carries `dropdown_open` snapshot from doc.ui
- Paint: dropdown overlay anchored under the open chip; popover
card with rounded border, per-value rows, active-value
highlight via theme.muted
- Hit-test: dropdown overlay takes top-most priority so a click
on a value row wins over the chip / row beneath
- Host wiring (`property_dispatch.rs`):
* AxisChip click toggles axis_dropdown_open (same chip closes,
different chip switches)
* AxisDropdownItem click calls VariableTable::set_active_theme +
closes the dropdown + pushes a history snapshot
Test: `axis_dropdown_hit_routes_to_named_value` asserts that with
themes=[mode: light/dark/system] and the dropdown open, hits on
rows 0 and 2 return the correct (axis, value) pair.
377 shell-core tests pass (was 376).
79cf73df19