Add Coherent FieldMaster GS power meter driver

[dccote]

Summary

Adds a driver for the Coherent FieldMaster GS laser power/energy meter, which connects over RS-232 through a generic FTDI adaptor. Verified end-to-end against real hardware (reads ~4.6 W from an LM-10 thermopile head).

• FieldMasterDevice — PowerMeterDevice subclass driven through the library's SerialPort primitives.
• DebugFieldMasterDevice — hardware-free stub (classIdVendor=0xFFFF, classIdProduct=0xFFF1).
• tests/testFieldMasterDevice.py — 6 debug tests (always run) + 2 hardware tests (skipTest when the meter is absent / not on its Home screen).
• FieldMaster GS manual added under hardwarelibrary/manuals/.
• CHANGELOG.md entry under Unreleased.

Protocol (verified)

• 9600 baud, N/8/1, no handshaking — the meter wires only TxD/RxD/GND (3-wire).
• Commands: pw? (power, W), en? (energy, J), wv?/wv <m> (wavelength), v (version).
• Wavelength is exposed in nanometres (family convention) and converted to metres on the wire.

Two operational constraints worth knowing

1. Home screen only. The meter processes RS-232 only while on its Home or Trend screen; otherwise it silently buffers. doInitializeDevice probes on connect and raises with that hint. (Do not send it h/q — they navigate it off Home and wedge the serial firmware until a power cycle.)
2. Terminator is a Menu setting (LF / CR / CR-LF). Pass it as terminator= (default b'\n'); it is applied to port.terminator, which both reads and writes use, so they stay in sync with the meter's Menu.

Usage

from hardwarelibrary.powermeters import FieldMasterDevice

meter = FieldMasterDevice(serialNumber="FTFDLOTS")   # terminator=b'\r\n' if Menu is CR-LF
meter.initializeDevice()
print(meter.measureAbsolutePower())        # watts
print(meter.getCalibrationWavelength())    # nm
meter.shutdownDevice()

Test plan

• pytest hardwarelibrary/tests/testFieldMasterDevice.py → 6 passed, 2 skipped (no/absent hardware)
• Live read against the physical meter (LF and CR-LF terminators both read ~4.60 W; CR correctly refused since the unit's Menu is LF)

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