Add GnssStatusTel telemetry for the Blueye GNSS receiver

[jp-pino]

Summary

Adds a GnssStatusTel telemetry message backed by new GnssStatus, GnssSatellite, and GnssConstellation types so GNSS behaviour can be diagnosed from dive logs for the first time:

• Fix quality, satellites used / in view, HDOP
• Per-axis standard deviations (from GST), COG / SOG, global position
• Per-satellite SNR by constellation (GPS / GLONASS / Galileo / BeiDou)
• Driver health: connection state, baud rate, reconnect and receive-error counters, time since last sentence

Merge order

Part of the GNSS driver rework. This PR goes first; the p2_msgs, libguestport, and p2_drone PRs build on it.

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[follesoe]

First pass, superseded by the comment below once the scope widened to the whole GSA/geometry set. Kept for the reasoning that has not changed:

• elevation / azimuth use -1 for "not reported", which is safe here in a way a 0.0 standard deviation is not — neither is ever negative — and avoids proto3 optional where the rest of this repo uses none.
• is_accuracy_valid rather than switching libguestport's stddev sentinel to NaN: those deviations also feed gnss_rov_avg_stddev and avg_stddev_buffer in the position observer, where a NaN in a rolling variance is worse than a large finite number.

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[follesoe]

Reviewed against what the driver actually produces and extended in 1c25d36 + c562a3d. GnssStatus goes 16 → 31 fields; everything added is already parsed by libguestport, and used_in_fix needed one new sentence (GSA, BluEye-Robotics/libguestport#393).

Satellite geometry. GnssSatellite gains elevation, azimuth and used_in_fix. Without the first two a satellite is an id and a signal strength — enough for a bar chart, not for plotting the sky the receiver sees, and an obstructed horizon is invisible in the data. used_in_fix is the tracked vs contributing distinction that every GNSS tool draws as filled against hollow. Angles use -1 for "not reported": safe here in a way a 0.0 deviation is not, since neither is ever negative, and it avoids proto3 optional where the rest of this repo uses none.

Four validity bits, each load-bearing. These exist because the alternative is a UI that lies:

field	without it
is_used_in_fix_valid	a GSA with no system id can't be attributed to a constellation, and a client reads that as "nothing is in the fix"
is_accuracy_valid	stddev_for_status() maps the numeric_limits sentinel to 0.0F, so "no GST sentence yet" renders as ± 0.00 m
is_course_valid	RMC leaves the course empty when stationary, so a still drone reports the last heading it saw while moving. GnssPosition keeps this bit; GnssStatus dropped it
fix_mode ≠ UNSPECIFIED	no way to tell a 2D fix from a 3D one, which is what explains a wrong altitude

satellites_in_view did not mean satellites in view. It held last_report.satellites.size() — entries with a non-empty SNR seen in the last 30 s, always below the receiver's own count and lagging it. Field 5 keeps its number and type but is renamed satellites_tracked; satellites_in_view (17) now carries the count GSV reports on every sentence and the driver was discarding.

Also added, all previously parsed and dropped: pdop/vdop; the horizontal error ellipse (stddev_semi_major, stddev_semi_minor, error_ellipse_orientation), stddev_altitude and range_rms — two axis-aligned deviations cannot describe a tilted uncertainty, which is the normal shape with half the sky behind a quay wall; altitude, the cheapest sanity check on a surface fix, since multipath often keeps a respectable HDOP while putting the antenna hundreds of metres off vertically; and differential_age/differential_station, without which a fix quality of 2, 4 or 5 cannot be verified after the fact.

Skipped: GGA geoid_separation — near constant for a location, legitimately negative so it has no free sentinel, and altitude carries the diagnostic value.

protolint and protoc pass; CI green. Siblings updated to match: BluEye-Robotics/libguestport#393, BluEye-Robotics/p2_msgs#51, BluEye-Robotics/p2_drone#1039.

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