fix(parser): parenthesized sub-expressions in arithmetic (trim_outer shadowing bug)

parse_expr trimmed outer parens in two statements:
  let start = trim_outer(..).0;
  let end   = trim_outer(.., start, ..).1;   // uses the already-incremented start
so the second call saw a range no longer starting with '(' and left the closing ')'
in range -> every parenthesized expression became Expr::Unknown (RS0015). Collapse to
a single tuple destructure. Now (a+b), (a+b)*c, a*(b+c), ((a+b)*c)-a all parse+eval.
Regression fixture: tests/fixtures/pass/paren-grouping-arithmetic.rss. cargo test green.

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

Author: claude

.claude/scheduled_tasks.lock

@@ -0,0 +1 @@
+{"sessionId":"a5283d36-5897-48c1-b2a0-489912035d4c","pid":2925204,"procStart":"19492512","acquiredAt":1780991634075}

crates/rsscript/src/syntax/parser.rs

@@ -2217,8 +2217,7 @@ fn split_match_pattern_guard(tokens: &[Token], start: usize, end: usize) -> Opti
 }
 
 fn parse_match_pattern(tokens: &[Token], start: usize, end: usize) -> Option<MatchPattern> {
-    let start = trim_outer(tokens, start, end).0;
-    let end = trim_outer(tokens, start, end).1;
+    let (start, end) = trim_outer(tokens, start, end);
     if start >= end {
         return None;
     }
@@ -2441,8 +2440,7 @@ fn starts_like_constructor(name: &str) -> bool {
 }
 
 fn parse_expr(tokens: &[Token], start: usize, end: usize) -> Option<Expr> {
-    let start = trim_outer(tokens, start, end).0;
-    let end = trim_outer(tokens, start, end).1;
+    let (start, end) = trim_outer(tokens, start, end);
     if start >= end {
         return None;
     }

crates/rsscript/tests/fixtures/pass/paren-grouping-arithmetic.rss

@@ -0,0 +1,15 @@
+// Regression: parenthesized sub-expressions in arithmetic must parse (trim_outer
+// double-call bug left the closing paren in range, yielding RS0015 Unknown).
+fn compute(a: Int, b: Int, c: Int) -> Int {
+    let g = (a + b)
+    let h = (a + b) * c
+    let i = a * (b + c)
+    let j = ((a + b) * c) - a
+    let k = (a + b) / c
+    return ((g + h) + i) + ((j + k) + 0)
+}
+
+fn main() -> Unit {
+    Log.write(message: read Int.to_string(value: read compute(a: 2, b: 3, c: 4)))
+    return Unit
+}