feat: add XDR (Transducer Measurements) sentence parser

[amoutiers]

Summary

Adds support for XDR sentences (Transducer Measurements), resolving part of #54.

XDR is a generic container sentence widely used in marine instrumentation. It carries variable-length groups of transducer readings — barometric pressure, air temperature, rudder angle, humidity, battery voltage, engine data, and more — from a single sentence.

Format

$--XDR,a,x.x,a,c--c[,a,x.x,a,c--c]*hh<CR><LF>

Each measurement is a group of 4 fields (type, value, units, name), repeatable.

Implementation

• XdrData struct containing a heapless::Vec<XdrMeasurement, 8>
• XdrMeasurement struct with transducer_type (char), value (f64), units (char), name (ArrayString<16>)
• Full nom-based parser handling variable-length measurement groups
• Feature-gated behind XDR (included in the other category alongside HDT, MDA, MWV, TXT, ZDA)

Real-world edge cases handled

Tested against real NMEA logs from B&G, NKE, Airmar, Calypso, and SignalK sources:

• Empty name fields — e.g., $WIXDR,C,022.0,C,,*52 (Calypso anemometer)
• Empty unit fields — e.g., $IIXDR,G,200,,ENGHRS*3E (engine hours)
• Incomplete trailing quadruplets — parser stops gracefully instead of erroring
• Multiple measurements per sentence — up to 8 groups (B&G H5000 sends 5)
• Negative values — e.g., rudder angle A,-2.3,D,RUDDER
• Various talker IDs — II, WI, YX, SD, HC, ER all observed in the wild

Transducer types observed in practice

Type	Meaning	Common use
A	Angular displacement	Pitch, roll, heel, rudder
C	Temperature	Air temp, engine coolant
D	Depth	Hi/lo frequency sounder
E	Fluid level	Fuel, freshwater (v4.11)
G	Generic	Magnetic field, engine hours
H	Humidity	Relative humidity
I	Current	Battery amperes
P	Pressure	Barometric, oil pressure
T	Tachometer	Engine RPM
U	Voltage	Battery, alternator

Motivation

I'm building a sailing navigation app and needed XDR parsing for on-board weather stations and autopilot transducers. Figured I'd contribute it upstream rather than maintain a custom parser. Happy to follow up with other marine sentences (MWD, RSA, ROT, HDG, etc.) if there's interest.

Partially addresses #54.