feat: NL-42/NL-52 serial control layer (USB/RS-232, dependency-free)

Adds a stdlib-only (termios, no pyserial) control layer for the older RION
NL-42/NL-52 meters, which have no Ethernet option and speak the same ASCII
command family as the NL-43 over RS-232C or USB (virtual COM port).

- nl52/protocol.py: I/O-free parsers for DOD? (14 fields) and DRD? (9
  fields), result codes, and level tokens. Handles '--.-' disabled-channel
  nulls and space-padded fixed-width fields.
- nl52/client.py: termios SerialPort + NL52Client. Echo-tolerant result-code
  reading, buffered multi-line reads, NL-52 rate limits (200ms; 1s for DOD?).
  is_request = "?" in cmd (NL-52 requests can carry a param after '?', e.g.
  'System Version?NL').
- nl52/cli.py: bench tool — probe/status/start/stop/monitor/poll/raw — for
  testing over USB on the dev server with nothing to install.
- test_nl52_protocol.py: 45 parser assertions, host-runnable (no hardware).
  Client exchange logic separately validated via pty loopback.

NL-52 quirks vs NL-43: DOD has no Lpeak and no measurement-state (query
Measure? separately); DRD streaming requires the NX-42EX option.

Not yet: SLMM integration (async transport / serial->TCP bridge for the RX55
PAD-mode path), DB model, validation against real hardware.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nl52/__init__.py

@@ -0,0 +1,23 @@
+"""
+RION NL-42 / NL-52 serial control layer.
+
+The NL-52 has no Ethernet option (unlike the NL-43's NX-43EX LAN card); it
+speaks the same ASCII command family over RS-232C or USB (virtual COM port).
+This package provides a dependency-free serial client + parser so the meter
+can be driven directly over USB on the bench, and later bridged onto the
+network via the RX55's serial PAD mode (or a ser2net host).
+
+Modules:
+    protocol  — pure command/response parsing (no I/O, unit-testable)
+    client    — termios-based serial transport + NL52Client command methods
+    cli       — command-line tool for bench testing over USB
+"""
+
+from nl52.protocol import (  # noqa: F401
+    DODSnapshot,
+    DRDSample,
+    ResultError,
+    parse_dod,
+    parse_drd,
+    RESULT_CODES,
+)