replace ptyc with forkpty() via Dart FFI

NativePty calls forkpty() directly — no helper binary, no socketpair,
no SCM_RIGHTS. The master fd stays in-process. Reader isolate uses
poll() for clean shutdown.

Based on the pty-spike proof-of-concept. Platform-aware: macOS uses
libSystem (DynamicLibrary.process), Linux needs libutil.so.1.
TIOCSWINSZ platform-detected.

PaneRegistry updated to use NativePty. registerPaneCommands no longer
needs a Toolchain parameter. All ptyc references removed from the
daemon layer.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/backend_entry.dart

@@ -98,7 +98,7 @@ void backendEntry(BackendBootMessage boot) {
       registerPqlCommands(dispatcher, pql);
 
       final paneRegistry = PaneRegistry(events: eventSink);
-      registerPaneCommands(dispatcher, paneRegistry, toolchain: toolchain);
+      registerPaneCommands(dispatcher, paneRegistry);
 
       // Tell the frontend the project is active.
       frontendPort.send({