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>

test/terminal/terminal_test.dart

@@ -6,6 +6,8 @@
 /// dependency.
 library;
 
+import 'dart:convert' show utf8;
+
 import 'package:clide/src/terminal/src/core/buffer/cell_offset.dart';
 import 'package:clide/src/terminal/src/core/input/keys.dart';
 import 'package:clide/src/terminal/src/core/mouse/button.dart';
@@ -121,6 +123,29 @@ void main() {
       t.write('\x1b[31mred'); // SGR red foreground
       expect(t.cursor.foreground, isNot(0)); // foreground was set
     });
+
+    // T-373: byte consumers used to utf8.decode per chunk — a rune split
+    // across PTY reads rendered as U+FFFD garbage.
+    test('writeBytes joins a multi-byte rune split across two calls', () {
+      final t = _Recorder().build();
+      final euro = utf8.encode('€'); // 3 bytes: E2 82 AC
+      t.writeBytes(euro.sublist(0, 1));
+      t.writeBytes(euro.sublist(1));
+      expect(t.buffer.lines[0].getCodePoint(0), '€'.codeUnitAt(0));
+    });
+
+    test('writeBytes decodes consecutive whole chunks like write', () {
+      final t = _Recorder().build();
+      t.writeBytes(utf8.encode('héllo'));
+      final line = [for (var i = 0; i < 5; i++) t.buffer.lines[0].getCodePoint(i)];
+      expect(String.fromCharCodes(line), 'héllo');
+    });
+
+    test('writeBytes with an empty chunk is a no-op', () {
+      final t = _Recorder().build();
+      t.writeBytes(const []);
+      expect(t.buffer.cursorX, 0);
+    });
   });
 
   group('Terminal — keyInput', () {