Add a "Best DX" card to the logbook Stats tab (furthest station worked)

[patrickrb]

What & why

Adds a Best DX highlight card to Logbook → Stats, showing the furthest station the operator has worked — callsign, distance (in their preferred mi/km), and grid — measured great-circle from their configured Maidenhead grid.

Seeing "what's the furthest I've ever worked?" is one of the first things an FT8/DX operator wants from a logbook, and it's a natural brag stat next to Total QSOs / DXCC / Zones. Every logged QSO already stores a grid, so this surfaces information the app already has — no new data, no schema change, no extra DB query (it reduces over the records already loaded for the Stats tab).

How it looks

The card sits right below the four big stat cards (Total QSOs / DXCC / CQ / ITU), before the band distribution donut:

BEST DX
┌───────────────────────────────────────┐
│  VK3ABC                     16,842 km  │
│  QF22                                  │
└───────────────────────────────────────┘

It only appears once there's something measurable — i.e. the operator's own grid is configured and at least one logged QSO has a parseable grid. No grid set → the section is hidden (no empty header).

Implementation

• computeBestDx(records, distanceKm) — a pure, top-level internal reducer that takes an injected grid→distance function and returns the furthest BestDx(callsign, grid, distanceKm). It skips records with a blank callsign/grid, an unparseable grid, or a non-positive/NaN distance (the last mirrors MaidenheadGrid's 0-km result for two stations sharing a grid — a distance floor, never a "best DX").
• The Composable BestDxCard is a thin wrapper; the screen passes a MaidenheadGrid.getDist(myGrid, …)-backed lambda and renders the distance with MaidenheadGrid.formatDist (unit-aware).
• Follows the existing extract-pure-helper pattern already used for gridSquaresWorked / workedGridFields, so the decision logic is unit-testable without Robolectric.

Tests

New BestDxTest (7 plain-JVM cases): furthest wins, empty log → null, unparseable/blank grids skipped, blank callsign skipped, nothing-measurable → null, 0 km and NaN distances skipped, callsign/grid trimmed.

Verification

• ./gradlew :app:testDebugUnitTest --tests 'radio.ks3ckc.ft8af.ui.logbook.*' — green (BestDxTest 7/7, no regressions in the logbook suite).
• ./gradlew :app:assembleDebug — BUILD SUCCESSFUL (full native + APK).

Scope / compatibility

Display-only addition to the Stats tab. No change to decoding, transmit, CAT, or the FT8 protocol path. One new string (log_section_best_dx); other locales fall back to the default, consistent with recent stats additions.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.76%. Comparing base (e662cd7) to head (fd0b64d).

Files with missing lines	Patch %	Lines
...lin/radio/ks3ckc/ft8af/ui/logbook/LogbookScreen.kt	16.66%	38 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                dev     #673      +/-   ##
============================================
+ Coverage     36.73%   36.76%   +0.03%     
- Complexity      197      211      +14     
============================================
  Files           216      219       +3     
  Lines         26882    27102     +220     
  Branches       3284     3332      +48     
============================================
+ Hits           9876     9965      +89     
- Misses        16781    16901     +120     
- Partials        225      236      +11     

Flag	Coverage Δ
android	15.39% <16.66%> (+0.35%)	⬆️
native	9.93% <ø> (ø)

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Files with missing lines	Coverage Δ
...lin/radio/ks3ckc/ft8af/ui/logbook/LogbookScreen.kt	7.03% <16.66%> (+0.43%)	⬆️

... and 13 files with indirect coverage changes

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