Add static variable resolution with scope tracking

- Add distance-based get_at on EnvironmentStack
- Add distance-based assign_at on EnvironmentStack
- Introduce ErrorSink to accumulate diagnostics across passes
- Implement Resolver with a ScopeStack and per-node distance map
- Extend interpreter to store locals distances for runtime lookup
- Add tests for resolution behavior and error accumulation
  Add static variable resolution with scope tracking

src/common/lox_result.rs

@@ -479,3 +479,90 @@ pub fn io_error<T>(message: impl Into<String>) -> LoxResult<T> {
         message: message.into(),
     })
 }
+
+/// Accumulates diagnostics during a pass instead of bailing on the first one.
+///
+/// Static analyses (e.g. the resolver) `report` problems and keep traversing so
+/// the user sees every error at once, then surface them at the end.
+#[derive(Debug, Default)]
+pub struct ErrorSink {
+    errors: Vec<LoxError>,
+}
+
+impl ErrorSink {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Record a diagnostic and keep going.
+    pub fn report(&mut self, error: LoxError) {
+        self.errors.push(error);
+    }
+
+    pub fn has_errors(&self) -> bool {
+        !self.errors.is_empty()
+    }
+
+    pub fn errors(&self) -> &[LoxError] {
+        &self.errors
+    }
+
+    /// Merge another sink's errors into this one (e.g. from a separate pass).
+    pub fn extend(&mut self, other: ErrorSink) {
+        self.errors.extend(other.errors);
+    }
+
+    pub fn into_errors(self) -> Vec<LoxError> {
+        self.errors
+    }
+
+    /// Bridge to the single-error [`LoxResult`] API: `Ok` if empty, otherwise
+    /// the first reported error.
+    pub fn into_result(self) -> LoxResult<()> {
+        match self.errors.into_iter().next() {
+            Some(error) => Err(error),
+            None => Ok(()),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::frontend::source_registry::SourceSlice;
+
+    fn parse_err(message: &str) -> LoxError {
+        LoxError::ParseError {
+            source_slice: SourceSlice::synthetic(),
+            message: message.to_string(),
+        }
+    }
+
+    #[test]
+    fn empty_sink_is_ok() {
+        let sink = ErrorSink::new();
+        assert!(!sink.has_errors());
+        assert!(sink.into_result().is_ok());
+    }
+
+    #[test]
+    fn accumulates_multiple_errors() {
+        let mut sink = ErrorSink::new();
+        sink.report(parse_err("first"));
+        sink.report(parse_err("second"));
+        assert!(sink.has_errors());
+        assert_eq!(sink.errors().len(), 2);
+        assert!(sink.into_result().is_err());
+    }
+
+    #[test]
+    fn extend_merges_sinks() {
+        let mut a = ErrorSink::new();
+        a.report(parse_err("a"));
+        let mut b = ErrorSink::new();
+        b.report(parse_err("b1"));
+        b.report(parse_err("b2"));
+        a.extend(b);
+        assert_eq!(a.into_errors().len(), 3);
+    }
+}