refactor(lower): use nullable pattern vocabulary

Author: zharinov

crates/plotnik-lib/src/compiler/lower/thompson/patterns.rs

@@ -439,13 +439,6 @@ impl NfaBuilder<'_> {
         self.ctx.analysis.pattern_facts.pattern_is_nullable(pattern)
     }
 
-    /// A sequence item that may consume nothing: a skippable quantifier, a
-    /// reference to a nullable definition, a group of such items, or an
-    /// alternation with a nullable alternative.
-    pub(super) fn is_skippable_item(&self, pattern: &Pattern) -> bool {
-        self.pattern_is_nullable(pattern)
-    }
-
     /// [`pattern_owns_iteration`] extended to nullable references: the inlined
     /// body stands in for the item, so iteration ownership is the body's — a
     /// quantifier-rooted body owns its sibling search, and wrapping it in a

crates/plotnik-lib/src/compiler/lower/thompson/sequences.rs

@@ -187,7 +187,7 @@ impl NfaBuilder<'_> {
         let first_is_skippable = nav_modes
             .first()
             .and_then(|(idx, _)| items[*idx].as_pattern())
-            .is_some_and(|p| self.is_skippable_item(p));
+            .is_some_and(|p| self.pattern_is_nullable(p));
         // The skip path makes the follower the new "first" item, so it must re-derive
         // first-position navigation rather than the sibling `Next` the match path uses.
         // This is required whenever the first item navigates to a position (`Down*` into
@@ -225,7 +225,7 @@ impl NfaBuilder<'_> {
         let last_is_skippable = nav_modes
             .last()
             .and_then(|(idx, _)| items[*idx].as_pattern())
-            .is_some_and(|p| self.is_skippable_item(p));
+            .is_some_and(|p| self.pattern_is_nullable(p));
 
         // Build chain in reverse: last pattern exits to `exit`, each prior exits to next.
         let mut current_exit = exit;