revert: drop PLOTWINDOW forced endpoint snap (#293)

Remove the forces_endpoint_snap mechanism merged in #293:
CadCommand::forces_endpoint_snap, Snapper::snap_forced_corners /
forced_corner_modes, the PlotWindowCommand override, the on_tick +
click-commit wiring, and the associated tests. PLOTWINDOW now uses the
normal snapper like every other command.

Rationale: forcing Endpoint snap on regardless of the global OSNAP master
overrides an explicit user setting and is surprising — a plot window is a
coarse print region that usually wants a margin, not a corner lock, and
the forced set only caught Endpoint (not center/mid/intersection) so it
was a partial, opinionated helper. Parking it pending a design discussion
with the PR author; can be reinstated behind a clearer UX (e.g. respecting
OSNAP, or an opt-in modifier) if justified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/command.rs

@@ -530,13 +530,6 @@ pub trait CadCommand: Send {
         false
     }
 
-    /// True if this command should have Endpoint/Intersection object-snap
-    /// forced on while it is active, regardless of the global OSNAP toggle.
-    /// Used by PLOTWINDOW so the plot-area corners always snap to geometry.
-    fn forces_endpoint_snap(&self) -> bool {
-        false
-    }
-
     /// Called when the text editor closes, either because the user committed or cancelled the edit.
     fn on_editor_closed(&mut self, _committed: bool) -> CmdResult {
         CmdResult::Cancel