add Terminal.writeBytes with a persistent UTF-8 decoder (T-373)

The terminal's only ingestion API was write(String), so both byte
consumers decoded per chunk — a multi-byte rune split across PTY
reads (or a tail window starting mid-character, which FileTailFollower
does by construction) rendered as U+FFFD garbage. writeBytes feeds a
per-instance chunked Utf8Decoder that carries partial-rune state
across calls; the terminal pane and the Bash live-tail follower now
use it, and write(String) stays for tests and programmatic writes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/builtin/claude/src/conversation_view.dart

@@ -463,7 +463,9 @@ class _BashLiveTailState extends State<_BashLiveTail> {
     if (source == null) return; // no file-backed source → muted note in build
     final term = Terminal(maxLines: 1000);
     _terminal = term;
-    _follower = FileTailFollower(source, onData: (bytes) => term.write(utf8.decode(bytes, allowMalformed: true)));
+    // writeBytes: the follower's chunk boundaries are arbitrary (it can even
+    // start mid-rune by construction) — keep decode state across reads (T-373).
+    _follower = FileTailFollower(source, onData: term.writeBytes);
     unawaited(_follower!.start());
   }
 

lib/builtin/terminal/src/terminal_pane.dart

@@ -114,8 +114,9 @@ class _TerminalPaneState extends State<TerminalPane> {
         case 'pane.output':
           final b64 = event.data['bytes_b64'];
           if (b64 is String) {
-            final bytes = base64Decode(b64);
-            _terminal.write(utf8.decode(bytes, allowMalformed: true));
+            // writeBytes keeps UTF-8 decode state across chunks — a rune
+            // split across PTY reads must not become U+FFFD (T-373).
+            _terminal.writeBytes(base64Decode(b64));
           }
         case 'pane.exit':
           setState(() => _error = 'Shell exited.');