TX: add "Hold TX freq" setting (WSJT-X-style)

[patrickrb]

Problem

Tester feedback: "transmission->TX/RX split option feeling invert. when I turn on it, I expect holding my freq and move RX freq only, but it move TX offset to QSO target."

In same-frequency (TX/RX split, synFrequency) mode, setTransmit() moves your TX offset onto the frequency of the station you answer (FT8TransmitSignal.java:328). There was no way to keep your own TX offset.

Change

New holdTxFreq setting (default off, persisted as "holdTxFreq", with a toggle in Transmission settings right under TX/RX split). When on, your TX offset is held when answering a station instead of moving to theirs — the WSJT-X "Hold Tx Freq" behavior.

The follow now goes through shouldFollowTargetFreq(synFrequency, holdTxFreq) = synFrequency && !holdTxFreq, so:

• split off → never follows (unchanged),
• split on, hold off → follows the target (unchanged default),
• split on, hold on → holds your TX offset (new).

Default off keeps existing behavior identical until the operator opts in. Hound/DXpedition auto-QSY is untouched.

Scope note

This is the minimal "add a setting to hold TX" the tester asked for. The broader waterfall-tap semantics rework they also mused about ("change TX offset only whenever option activated or not") is intentionally not included here.

Tests

• New shouldFollowTargetFreq() predicate + 3 unit tests in FT8TransmitSignalTest.
• testDebugUnitTest green; built and installed on the Pixel 8, launches cleanly.

Part of a series addressing tester feedback on the Android UI (the TX/RX split item).

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 11.45%. Comparing base (6bc083c) to head (6941f50).
⚠️ Report is 29 commits behind head on dev.

Files with missing lines	Patch %	Lines
...o/ks3ckc/ft8af/ui/settings/TransmissionSettings.kt	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                dev     #352      +/-   ##
============================================
- Coverage     11.70%   11.45%   -0.26%     
- Complexity      113      120       +7     
============================================
  Files            85       89       +4     
  Lines         11691    12575     +884     
  Branches       2110     2248     +138     
============================================
+ Hits           1369     1441      +72     
- Misses        10191    11003     +812     
  Partials        131      131              

Files with missing lines	Coverage Δ
...o/ks3ckc/ft8af/ui/settings/TransmissionSettings.kt	0.00% <0.00%> (ø)

... and 3 files with indirect coverage changes

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[Copilot]

Pull request overview

Adds an optional WSJT-X-style “Hold TX freq” setting so operators can keep their TX offset when answering a station, instead of automatically moving TX onto the target’s frequency in synFrequency mode.

Changes:

• Introduces holdTxFreq (default off), persists it via config ("holdTxFreq"), and exposes it in Transmission settings UI.
• Refactors the “follow target frequency” decision into shouldFollowTargetFreq(synFrequency, holdTxFreq) and applies it in setTransmit().
• Adds unit tests covering the new predicate behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
ft8af/app/src/test/java/com/k1af/ft8af/ft8transmit/FT8TransmitSignalTest.java	Adds unit tests for shouldFollowTargetFreq() decision logic.
ft8af/app/src/main/res/values/strings_compose.xml	Adds new user-facing strings for the Hold TX frequency setting.
ft8af/app/src/main/kotlin/radio/ks3ckc/ft8af/ui/settings/TransmissionSettings.kt	Adds the Hold TX frequency toggle and persists it to config.
ft8af/app/src/main/java/com/k1af/ft8af/GeneralVariables.java	Adds global holdTxFreq setting flag.
ft8af/app/src/main/java/com/k1af/ft8af/ft8transmit/FT8TransmitSignal.java	Uses shouldFollowTargetFreq() in setTransmit() and adds the predicate for testability.
ft8af/app/src/main/java/com/k1af/ft8af/database/DatabaseOpr.java	Loads holdTxFreq from persisted config at startup.

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