serial: add pty sink (mode = "pty")

[codewiz]

Adds a pseudo-terminal serial sink: [serial] mode = "pty" (or --serial pty).
The emulator allocates a host pty, logs the slave path (/dev/pts/N), and
bridges Paula's serial port to it, so a terminal program attaches directly with
no network in the loop -- with an AUX: shell on the Amiga side, a local
AmigaDOS console.

serial: pty at /dev/pts/7 (connect with e.g. "minicom -D /dev/pts/7", "screen /dev/pts/7" or "cu -l /dev/pts/7")

What's here

• PtySerialSink (Unix only): posix_openpt/grantpt/unlockpt/ptsname,
  slave line discipline set raw (no echo, no CR/LF rewriting) so the Amiga gets
  bytes verbatim. Mirrors TcpSerialSink: a background reader thread stages
  input into a channel and a signed counter drives has_pending_input, so
  Paula's idle fast path polls a counter, never a syscall.
• The sink holds its own slave fd open for its lifetime. That keeps the pty
  alive across terminal programs attaching/detaching, and stops the master read
  from returning EIO (the "no slave attached" error a naive blocking reader
  would hot-spin on) while nothing is attached.
• Config/CLI: SerialMode::Pty, parse_serial_mode, launcher picker + label,
  build_serial_sink (errors clearly on non-Unix hosts). copperline.example.toml
  documents pty (and fills in the previously-undocumented tcp mode).

Testing

• Unit test pty_sink_round_trips_input_and_output: opens the sink, attaches to
  the slave path, checks input staging and output delivery both directions.
• Manual: booted an A1200/68040 image with --serial pty, ran an AUX: shell
  over the pty, drove it interactively from minicom/screen/picocom.

Not in this PR (follow-up): an opt-in output-cooking option (0x9B C1 CSI ->
7-bit ESC[, \n -> \r\n) so dumb/UTF-8 terminals render the Amiga console
without per-terminal termios tuning. Kept separate to keep this change small.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a Unix pseudo-terminal (“pty”) serial sink so the emulator can bridge Paula’s serial port directly to a host /dev/pts/N, enabling local terminal attachment without TCP.

Changes:

• Add PtySerialSink (Unix-only) with a background reader thread and has_pending_input counter, mirroring the existing TCP sink pattern.
• Extend serial mode plumbing across config parsing, CLI/help text, emulator sink construction, and launcher UI labels to support pty.
• Document tcp and pty serial modes in copperline.example.toml.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/serial.rs	Implements the new PtySerialSink and adds a unit test for pty round-tripping.
src/config.rs	Adds SerialMode::Pty, label support, and parsing/error-message updates.
src/emulator.rs	Wires SerialMode::Pty into build_serial_sink with Unix/non-Unix handling.
src/main.rs	Updates --serial CLI error/help text to include tcp/pty.
src/video/launcher.rs	Adds Pty to the launcher’s serial-mode picker and label mapping.
copperline.example.toml	Documents tcp and pty serial configuration options.

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[LinuxJedi]

Another great feature. Thanks again!