Parser: audio statements (play / start_music / stop_music)

The \`play\`, \`start_music\`, and \`stop_music\` keywords were lexed
but not parsed. Programs using them failed with a generic "unexpected
token" error. Now:

- \`Statement::Play(sfx_name, span)\`
- \`Statement::StartMusic(track_name, span)\`
- \`Statement::StopMusic(span)\`

parse successfully and flow through analyzer / IR lowering / codegen
as no-ops. Semantics match the spec but produce no output — no audio
driver exists yet. Users who need sound can still wire in custom
code via \`asm { ... }\` blocks.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/parser/mod.rs

@@ -899,6 +899,23 @@ impl Parser {
                 Ok(Statement::Scroll(x, y, span))
             }
             TokenKind::KwDebug => self.parse_debug_statement(),
+            TokenKind::KwPlay => {
+                let span = self.current_span();
+                self.advance();
+                let (name, _) = self.expect_ident()?;
+                Ok(Statement::Play(name, span))
+            }
+            TokenKind::KwStartMusic => {
+                let span = self.current_span();
+                self.advance();
+                let (name, _) = self.expect_ident()?;
+                Ok(Statement::StartMusic(name, span))
+            }
+            TokenKind::KwStopMusic => {
+                let span = self.current_span();
+                self.advance();
+                Ok(Statement::StopMusic(span))
+            }
             TokenKind::KwAsm => {
                 let span = self.current_span();
                 self.advance(); // KwAsm