fix(spline): terminate on finish and preview the real curve (#282)

on_enter/on_escape returned CmdResult::CommitEntity, which commits the
spline but keeps the command armed with its accumulated points — the
cursor stayed in spline mode and the next click began a new spline,
trapping the user in the command loop. Return CommitAndExit so Enter/Esc
finalize the spline and return to the selection state.

The in-progress preview drew only the straight control polygon. Sample
the actual degree-3 clamped B-spline (same control points + knot vector
build() commits) so the rubber-band traces the exact final curve.

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

src/app/command_driver.rs

@@ -221,7 +221,7 @@ impl OpenCADStudio {
                 // A line/arc drawn by a repeating command advances the ARC_CONT
                 // continuation anchor, so ending one run and launching another
                 // keeps continuing from the last segment (mirrors the
-                // CommitAndExit arm). Non-continuable repeats (point/ray/spline)
+                // CommitAndExit arm). Non-continuable repeats (point/ray)
                 // leave the anchor untouched. (#327)
                 if matches!(
                     entity,