Phase 2 of the grip overhaul: when the cursor dwells on a selected
entity's grip the overlay opens a popup menu with entity-specific
options (Stretch / Add Vertex / Remove Vertex / Convert to
Arc-Line / …). Picking an item dispatches `apply_grip_menu`; the
default `Stretch` entry is a no-op that lets the user click the
grip to start the normal drag.
- `GripMenuItem` + `GripMenuAction` (full vocabulary covering
stretch, add/remove vertex, convert to arc/line, reverse arrows,
text-position variants, hatch parameters, tangent direction).
- `Grippable` trait gains `grip_menu` + `apply_grip_menu` with
trait-default `Stretch`-only impls so every existing entity gets
the popup for free.
- `EntityTypeOps` dispatches both methods across the same entity
list as the existing grip / property machinery.
- `OpenCADStudio` carries `grip_hover` (dwell tracker) +
`grip_popup` (open menu). `update_grip_hover` runs from
`ViewportMove`: hit-tests grips, refreshes the dwell timer, and
opens the popup after 600 ms. Cursor drift past 80 px or a grip
click dismisses it.
- `view.rs` paints the popup as an absolute-positioned column of
buttons at the grip's screen anchor; each button publishes
`Message::GripMenuPick(idx)`.
- `LwPolyline` overrides `Grippable` manually (skips the standard
macro) so its vertex grip exposes Add/Remove Vertex and its
segment-midpoint grip exposes Add Vertex + Convert to Arc/Line.
`apply_grip_menu` performs the edits: vertex insertion at the
chord midpoint inheriting the previous bulge, vertex removal,
and bulge toggle for the arc/line conversion.
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