Send GNSS quality (DDI 514) to implements

[gunicsba]

Summary

Second of four PRs split out of #73 (see #82 for the first). Parses the fix-quality byte from AOG's GPS/IMU PGN 0xD6 (source 0x7C) and pushes it to every ISOBUS client whose device description declares DDI 514 (GNSSQuality) as settable, every 250 ms. A non-settable declaration is skipped, since it cannot accept a Value command.

Deliberately not gated on track-control negotiation. In #73 this was only sent after the DDI 505/506 handshake completed. Nothing in the ISO DDI definitions ties DDI 514 to 505/506 — it's a general positioning signal that happens to share the TRACK DDI number block — so any implement that declares it gets it.

• AOG fix values 0-8 are forwarded unchanged: they are the NMEA 2000 GNSS Method values DDI 514 uses, including 6=Estimated, 7=Manual, 8=Simulated. Values above 8 and a stale/missing 0xD6 map to 1 (weakest real fix), not 0: some implements gate TRACK/section control on GNSS quality being non-zero, and AOG doesn't always send 0xD6 (e.g. Simulator mode).
• DDI 514 is mapped to its element in request_measurement_commands() next to the existing DDI mappings. Mapping only, no OnChange subscription — it's a value the TC pushes, not one it reads back. Like the existing mappings, this only runs for clients with sections (implements, not tractors).
• Docs: docs/PROTOCOL.md (source 0x7C, PGN 0xD6, outbound DDI 514) and the readme.

Keeping the follow-up PRs conflict-free

Trial-merged against #82 (TECU): clean, no conflicts. The new members in app.hpp and the new blocks in app.cpp are deliberately placed away from where #82 inserts. The readme gets its own "Guidance data sent to implements" subsection so the remaining guidance PRs append bullets there rather than all editing the shared capabilities list.

Test plan

• Release build + AOG_TC_VALIDATE_IOP=ON build, ctest 5/5.
• Real-bus check that an implement declaring DDI 514 receives the value — not yet run against hardware.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟡 Changes recommended

Valid GNSS methods 6–8 are misreported, and non-settable DDI declarations may receive unsupported commands.

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Review effort: Balanced
Findings: 2 · 3

Open (5)

• Preserve DDI 514 GNSS method values 0–8 · New
• Only map DDI 514 when the process data is settable · New
• Document and preserve standardized GNSS method values 6–8 · New
• Document valid DDI 514 values 6–8 · New
• Complete API documentation for DDI 514 values 6–8 · New

What changed in this PR

Adds periodic GNSS quality forwarding from AOG to compatible ISOBUS implements.

Changes:

• Parses GNSS quality from PGN 0xD6.
• Maps and transmits DDI 514 every 250 ms.
• Documents the protocol and capability.

File	Description
src/​app.cpp	Parses and periodically forwards GNSS quality.
src/​task_controller.cpp	Maps DDI 514 and sends values.
include/​app.hpp	Adds GNSS quality state and timeout.
include/​task_controller.hpp	Exposes GNSS quality transmission.
docs/​PROTOCOL.md	Documents PGN 0xD6 and DDI 514.
readme.md	Lists GNSS quality support.

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