fix(snap): typed overrides yield to the prompt's options; one point-step gate

#1707 took any snap name typed at a point prompt as an override, so a
prompt offering Center, Tangent, Parallel or None (ARC, ELLIPSE, CIRCLE
TTR, ...) could no longer be answered with that option's name. An option
the prompt offers now wins. The point-step test it copied from MTP is one
helper shared by MTP and the override on both input paths.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/update/viewport_snap.rs

@@ -50,6 +50,35 @@ impl OpenCADStudio {
         self.accepted_snaps.clear();
     }
 
+    /// The active command is at a point prompt (keywords allowed) and not
+    /// picking an object: where MTP and typed snap overrides apply.
+    pub(crate) fn point_modifier_step(&self, i: usize) -> bool {
+        self.tabs[i].active_cmd.as_ref().is_some_and(|c| {
+            (!c.input_kind().wants_text() || c.point_step_accepts_keywords())
+                && !c.needs_entity_pick()
+        })
+    }
+
+    /// A snap override typed at a point prompt (`MID`, `END`, `NON`, ...). An
+    /// option the prompt offers wins, so `CEN` where Center is offered picks
+    /// Center.
+    pub(crate) fn typed_snap_override(
+        &self,
+        i: usize,
+        token: &str,
+    ) -> Option<crate::snap::SnapOverrideKind> {
+        let kind = crate::snap::parse_snap_override(token)?;
+        if !self.point_modifier_step(i) {
+            return None;
+        }
+        let typed = token.trim().trim_start_matches('_').to_uppercase();
+        let offered = self.tabs[i].active_cmd.as_ref()?.options().iter().any(|option| {
+            option.keyword.eq_ignore_ascii_case(&typed)
+                || option.label.to_uppercase().replace(' ', "").starts_with(&typed)
+        });
+        (!offered).then_some(kind)
+    }
+
     /// Apply a transparent one-shot snap override (from typed input or the context menu).
     pub(crate) fn apply_snap_override(&mut self, kind: crate::snap::SnapOverrideKind) {
         match kind {