fix(ghostty): make GhosttyTerminal.free() idempotent

ghostty 1.3's libghostty-vt dispatches effect callbacks via an
indirect call through the function table. Calling free() twice on
the same instance (e.g. manual wasmTerm.free() followed by
term.dispose()) drove ghostty_terminal_free into freed memory and
surfaced as a call_indirect signature mismatch on the second call.

Gate on this.handle at entry and zero it after the free.

Recovers the two #141 regression tests that started failing after
the merge to ghostty 1.3.

Author: sauyon

lib/ghostty.ts

@@ -1056,6 +1056,7 @@ export class GhosttyTerminal {
   }
 
   free(): void {
+    if (!this.handle) return;
     if (this.callbackRegistry) {
       this.callbackRegistry.instancesByHandle.delete(this.handle);
     }
@@ -1072,6 +1073,7 @@ export class GhosttyTerminal {
       this.renderHandle = 0;
     }
     this.exports.ghostty_terminal_free(this.handle);
+    this.handle = 0;
   }
 
   // ==========================================================================