fix: report null instead of fabricated zeros for unavailable location fields

[Lyokone]

Summary

Every platform had at least one field where "this fix does not carry that measurement" was indistinguishable from a real reading — of zero, due north, stationary, or perfectly accurate. LocationData has always documented these fields as double? / "will be null if not available"; this makes the native side actually honour that.

The iOS speed sentinel fix (#741) covered one field on one platform. This extends the same treatment to every scalar on every platform.

Before → after, per platform

	Was reported	Now
Android	0.0 from getSpeed()/getBearing()/getAccuracy()/… on a fix that carries no such field	key omitted, gated on the matching Location.hasX()
iOS/macOS	raw negative sentinel for horizontalAccuracy, verticalAccuracy, speedAccuracy, course; altitude always sent	nil; altitude follows verticalAccuracy, which is what Core Location documents as marking it invalid
Windows	heading/speed forced to 0.0; altitude always sent	omitted when the IReference<double> is null; altitude gated on AltitudeAccuracy, verticalAccuracy now forwarded
Linux	GeoClue2 sentinels forwarded verbatim — -1 speed/heading, altitude of -1.797e308	recognised and omitted; time now populated from Timestamp
Web	heading as NaN when stationary	non-finite readings normalised to null

Windows and Linux matter most in practice: without a GPS radio both position by Wi-Fi/IP, so every fix hit the bad path — Windows reported sea level, Linux reported an altitude 1.8e308 metres below the ellipsoid.

The web case is subtler than a fake zero. The Geolocation spec specifies heading as NaN, not null, when the device is stationary; since NaN != NaN, that made two otherwise identical LocationData compare unequal with unstable hashCodes.

Also included

• iOS/macOS: a fix whose horizontalAccuracy is negative is skipped rather than delivered — Core Location documents that as meaning the coordinate itself is invalid, and latitude/longitude are non-nullable, so it cannot be passed on without inventing one. didUpdateLocations takes the last valid fix in a batch, so this cannot drop an update that had a good fix available.
• Android: the NMEA mean-sea-level altitude is only applied to a fix that has an altitude to correct, and the NMEA caches are cleared when the listener detaches. It is a correction of the fix's own WGS 84 altitude, not a stand-in for a missing one, so a stale GPS reading could previously surface as a network-provider fix's current altitude.

Behaviour change to be aware of

isMock and isProducedByAccessory are now null when the platform exposes no such flag, instead of defaulting to false — "nothing checked" and "checked, and it was not mocked" are different claims. Android now always reports isMock; Apple reports both on iOS 15.0+/macOS 12.0+; Windows/Linux/web report null.

Nothing stops compiling — the affected fields were already nullable — but an app that force-unwraps (data.speed!, data.isMock!) or that treated the old 0 as a real reading will now see a null. Documented under ## Unreleased in all three changelogs; version bumps left as a separate commit per the usual flow.

Verification

Platform	Check	Result
Android	compileDebugKotlin + lintDebug	compiles; lint identical to baseline (4 errors / 7 warnings, no new NewApi)
iOS/macOS	swiftc -typecheck against the engine framework	clean
Linux	location desktop CI	clean, no compiler diagnostics
Windows	location desktop CI	clean, no compiler diagnostics
Dart/web	flutter analyze, 50 tests, dart format	clean

New tests cover the sparse-map path (all optionals null) and reported-0 vs absent flags, in both the double shape Android sends and the int shape Apple sends. One existing test asserted the old isProducedByAccessory default of false and was updated.

Not runtime-verified: GeoClue2's sentinel values and the AltitudeAccuracy-is-null-when-no-altitude behaviour are from the respective platform docs. Both compile, but the desktop workflow only builds — confirming them behaviourally needs a real Linux desktop and a Windows machine without GPS.