Language: match statement

match state {
        Title => { if button.start { state = Playing } }
        Playing => { /* ... */ }
        GameOver => { if button.a { state = Title } }
        _ => {}
    }

- Lexer: \`match\` keyword and \`=>\` (FatArrow) token
- Parser: \`parse_match\` after the existing loop constructs. Each
  arm is \`pattern => { body }\`, with \`_\` as the catch-all. The
  match scrutinee is parsed with struct-literal restriction enabled
  so the following \`{\` is unambiguously the match body, not a
  struct literal.
- The parser desugars match directly into an if/else-if chain so
  the analyzer, IR lowering, and codegen don't need new AST variants
  — each arm becomes \`scrutinee == pattern\` as the condition, and
  the default arm (if any) becomes the final \`else\` block.

Tests cover parse + full pipeline integration for state-style
dispatch using an enum.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

docs/language-guide.md

@@ -509,6 +509,37 @@ while i < 10 {
 }
 ```
 
+### Match Statement
+
+`match` matches a scrutinee against a sequence of patterns and
+executes the body of the first matching arm. Each arm's pattern is
+compared against the scrutinee with `==`. An underscore arm `_` acts
+as the catch-all:
+
+```
+enum State { Title, Playing, GameOver }
+var state: u8 = Title
+
+on frame {
+    match state {
+        Title => {
+            if button.start { state = Playing }
+        }
+        Playing => {
+            // ... game logic ...
+        }
+        GameOver => {
+            if button.a { state = Title }
+        }
+        _ => {}
+    }
+}
+```
+
+`match` desugars to an `if` / `else if` chain at parse time, so
+patterns can be any expression that produces a value comparable to
+the scrutinee.
+
 ### For Loop
 
 The `for` loop iterates over a half-open integer range `[start, end)`: