decouple Pane from PtySession so the web build compiles

Pane is now a pure data class — id, kind, pid, argv, cwd, title,
isClosed. The daemon-side PaneRegistry holds a parallel map of
PtySession keyed on id; registry methods look up both sides when
writing / resizing / closing.

The `clide.dart` barrel no longer re-exports `src/pty/*`,
`src/panes/registry.dart`, or the `*_commands.dart` modules — all
three transitively import `dart:ffi` which isn't available when
compiling to WebAssembly. The daemon entrypoint (bin/clide.dart) +
core tests import them via deep paths now. Pane / PaneKind /
DaemonEventSink / RecordingEventSink stay in the barrel since
they're pure data the Flutter app references over IPC.

Verified: `dart analyze` clean, 53 core tests green, 174 app tests
green, `make ui-smoke` compiles + serves + Playwright smoke passes,
daemon boots + ping round-trips + SIGTERMs cleanly.

Co-Authored-By: Claude <noreply@anthropic.com>

test/daemon/files_commands_test.dart

@@ -4,6 +4,7 @@ library;
 import 'dart:io';
 
 import 'package:clide/clide.dart';
+import 'package:clide/src/daemon/files_commands.dart';
 import 'package:test/test.dart';
 
 void main() {

test/daemon/pane_commands_test.dart

@@ -9,6 +9,8 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:clide/clide.dart';
+import 'package:clide/src/daemon/pane_commands.dart';
+import 'package:clide/src/panes/registry.dart';
 import 'package:test/test.dart';
 
 void main() {