feat(trim): TRIM/EXTEND options — edges, fence, crossing, edge, erase (#336)

Both commands gain a shared option mode machine:

- Cutting/Boundary edges (T/B): pick which objects act as the boundary
  (picked edges highlight yellow, the hovered candidate half-strength);
  Enter rebuilds the boundary set, empty = everything (quick mode).
- Fence (F): draw a dashed fence; every object crossing it is trimmed /
  extended at each crossing, re-applied to the surviving pieces so a
  fence across both overhanging ends clears both. Live red/cyan result
  preview while placing points.
- Crossing (C): crossing-selection semantics — the dashed green window
  picks objects crossing its edges AND wholly inside; runs through the
  fence machinery with a synthesized inside click.
- Edge (E): boundaries extrapolate (lines run past their ends, arcs
  close to full circles). Cuts that land on an implied extension draw a
  dashed yellow guide from the boundary's drawn end to the cut point,
  so the user sees which edge causes the cut. Extend previews also show
  the added tail as a bright dashed slice of the true curve.
- Erase (R, TRIM): picks delete without needing an intersection, with a
  red hover preview.
- Shift+click swaps trim<->extend per pick (new set_shift plumbing into
  the entity-pick and hover paths); the hover preview follows the held
  state. Project is out of scope for the 2D pipeline.

The per-type pick bodies moved to shared free fns (pick_trim_at /
pick_extend_at) used by direct picks, hover previews and the fence
pass; EXTEND's snapshot bookkeeping unified with TRIM's placeholder
flow. Option keywords now take precedence over the hex-handle reading
in feed_active_cmd ("F"/"C"/"E" are valid hex), and fence geos carry
sentinel handles — probing with NULL against NULL-handle geos excluded
the whole fence. Regression test: fence_trims_x_arm.

Closes #336

Co-Authored-By: Claude Fable 5 <[email protected]>

src/command.rs

@@ -1213,6 +1213,11 @@ pub trait CadCommand: Send {
     /// `ARC_CONT`) store it; most commands ignore it. Default no-op.
     fn set_ctrl(&mut self, _ctrl: bool) {}
 
+    /// Push the live Shift-key state into the command before each dispatch.
+    /// TRIM/EXTEND use it for the shift-select swap (Shift+click extends
+    /// during TRIM and trims during EXTEND, #336). Default no-op.
+    fn set_shift(&mut self, _shift: bool) {}
+
     /// Called when the user left-clicks in the viewport (point pick).
     fn on_point(&mut self, pt: DVec3) -> CmdResult;