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 <noreply@anthropic.com>

src/app/command_driver.rs

@@ -20,11 +20,13 @@ impl OpenCADStudio {
         }
         let i = self.active_tab;
         let ctrl = self.ctrl_down;
+        let shift = self.shift_down;
         let result: Option<CmdResult> = {
             let Some(cmd) = self.tabs[i].active_cmd.as_mut() else {
                 return Task::none();
             };
             cmd.set_ctrl(ctrl);
+            cmd.set_shift(shift);
             match input {
                 StepInput::Point(p) => Some(cmd.on_point(p)),
                 StepInput::Text(s) => cmd.on_text_input(&s),
@@ -176,6 +178,18 @@ impl OpenCADStudio {
             .as_ref()
             .is_some_and(|c| c.needs_entity_pick())
         {
+            // Option keywords take precedence over the handle reading —
+            // "F"/"C"/"E" are valid hex, but during TRIM/EXTEND they are the
+            // Fence/Crossing/Edge options (#336). Only an unconsumed token
+            // falls through to the handle interpretation.
+            let consumed = self.tabs[i]
+                .active_cmd
+                .as_mut()
+                .and_then(|c| c.on_text_input(token));
+            if let Some(r) = consumed {
+                let _ = self.apply_cmd_result(r);
+                return;
+            }
             if let Ok(v) = u64::from_str_radix(token.trim_start_matches("0x"), 16) {
                 let handle = Handle::new(v);
                 let pt = self.tabs[i]