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/ir/lowering.rs

@@ -344,6 +344,10 @@ impl LoweringContext {
             Statement::InlineAsm(body, _) => {
                 self.emit(IrOp::InlineAsm(body.clone()));
             }
+            Statement::Play(_, _) | Statement::StartMusic(_, _) | Statement::StopMusic(_) => {
+                // No audio driver yet — these parse but produce no
+                // IR.
+            }
         }
     }