feat: add record spread syntax { ...base, field: val }

Adds spread operator for records, allowing concise record updates:
  let p2 = { ...p, x: 5.0 }

Changes across the full pipeline:
- Lexer: new DotDotDot (...) token
- AST: optional spread field on Record variant
- Parser: detect ... at start of record expression
- Typechecker: merge spread record fields with explicit overrides
- Interpreter: evaluate spread, overlay explicit fields
- JS backend: emit native JS spread syntax
- C backend: copy spread into temp, assign overrides
- Formatter, linter, LSP, symbol table: propagate spread

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lexer.rs

@@ -90,6 +90,7 @@ pub enum TokenKind {
     Arrow,      // =>
     ThinArrow,  // ->
     Dot,        // .
+    DotDotDot,  // ...
     Colon,      // :
     ColonColon, // ::
     Comma,      // ,
@@ -181,6 +182,7 @@ impl fmt::Display for TokenKind {
             TokenKind::Arrow => write!(f, "=>"),
             TokenKind::ThinArrow => write!(f, "->"),
             TokenKind::Dot => write!(f, "."),
+            TokenKind::DotDotDot => write!(f, "..."),
             TokenKind::Colon => write!(f, ":"),
             TokenKind::ColonColon => write!(f, "::"),
             TokenKind::Comma => write!(f, ","),
@@ -373,7 +375,22 @@ impl<'a> Lexer<'a> {
                     TokenKind::Pipe
                 }
             }
-            '.' => TokenKind::Dot,
+            '.' => {
+                if self.peek() == Some('.') {
+                    // Check for ... (need to peek past second dot)
+                    // We look at source directly since we can only peek one ahead
+                    let next_next = self.source[self.pos..].chars().nth(1);
+                    if next_next == Some('.') {
+                        self.advance(); // consume second '.'
+                        self.advance(); // consume third '.'
+                        TokenKind::DotDotDot
+                    } else {
+                        TokenKind::Dot
+                    }
+                } else {
+                    TokenKind::Dot
+                }
+            }
             ':' => {
                 if self.peek() == Some(':') {
                     self.advance();