Fix ADSB altitude integer overflow and add on-ground indicator

[Copilot]

Per ICAO Annex 10 Vol IV §3.1.2.6.5.4, barometric altitude is decoded as 25N − 1000 ft, yielding valid negative values down to −1000 ft. Landing aircraft were hitting an unsigned cast overflow and a hard-clamp-to-zero, producing junk displays like 294967294 (screenshot above shows ANZ199L with this bug).

Changes

Integer overflow fix (ui_adsb_rx.cpp)

• Replace to_string_dec_uint((unsigned int)(entry.pos.altitude / 100), 4) with to_string_dec_int((int32_t)(entry.pos.altitude / 100), 4) — negative altitudes now display correctly (e.g. -1 for −100 ft) instead of wrapping to ~4 billion
• Remove the erroneous if (altitude < 0) altitude = 0; clamp in the DF 0/4/20 AC altitude decoder

On-ground indicator (ui_adsb_rx.cpp, ui_adsb_rx.hpp)

• Add bool on_ground{false} to AircraftRecentEntry
• Handle previously-ignored TC 5–8 (surface position) messages in DF 17: set on_ground = true, zero and invalidate the altitude field
• Clear on_ground when TC 9–22 (airborne position) is received
• Display GND (4 chars, matching column width) in the altitude column when on ground, and use altitude 0 for map color calculation to avoid stale airborne color after landing

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Proof that your changes work

🖥️ Proof it compiles

Pending — build log to be attached.

📱 Proof of testing on a real device

Pending — to be tested at an airfield with aircraft landing near/below sea level.

📡 Proof against a real emitter/receiver (if applicable)

Applicable — ADSB RX decoding. Pending real-device ADSB capture showing corrected altitude values for low-altitude aircraft.

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📚 Wiki documentation commitment

• I will (or already) create(d) wiki document for my newly added feature

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Checklist

• Kept changes minimal and limited to necessary files
• Verified functionality remains intact and code compiles
• Attached proof that the code compiles successfully (or marked N/A as a trusted contributor)
• Attached proof of testing on real PortaPack hardware (or marked N/A as a trusted contributor)
• Attached proof of testing against a real emitter/receiver (if RF-related), or marked N/A with justification
• I understand that by getting this PR merged, I am implicitly agreeing to create or update the corresponding wiki page (including a main-screen screenshot, description, controls, and limitations)
• I own all rights to this code (i.e., all code contained in this PR), including compliant usage rights for third-party libraries, and I agree that this code is licensed under the license of this project (GPL-3.0).
• If any third-party libraries are used, I confirm that their licenses comply with the requirements for contributing to this repository.
• Reviewed the Contributing Guidelines

[zxkmm]

@copilot no need to have all these and just for showing a GND. I don’t need “GND” and idk why you came up with this idea. Just prevent overflow, display data as is, even negative data.

[zxkmm]

Ok

[gullradriel]

Good to go, thanks @zxkmm for the review ;-)

[zxkmm]

Np, sorry for pinging, no idea why but I cannot approve this, maybe it’s because I asked copilot to make PR

[gullradriel]

That's it. You cannot review a PR you provided or contributed ;-)