Build with Gradle in two modules, and free core of JavaFX

Etappen 1 and 2 of the Compose migration (43 commits).

The Maven build, the single source tree and the JPMS module-info went together.
core now holds the domain and app-desktop the user interface, both built by
Gradle, packaged through jpackage from Gradle, with SpotBugs and PMD wired in.
JavaFX moves to the classpath, which is why the launcher is Main and not the
Application subclass. The PR check, the nightly artifacts, the release and the
AUR packages all build from Gradle; the macOS DMG points at its output.
Verified on Linux, macOS and Windows.

core then loses JavaFX entirely. It observes through kst4contest.observe --
ObservableValue, ObservableRoster, and UiDispatcher as the one boundary a
worker thread crosses to touch the interface. The controller collections become
rosters and the visible station list a pure derivation rather than a
FilteredList/SortedList chain, which is also what fixed two focus defects only
a load test had exposed: the keyboard focus stays where the operator put it,
and the send field keeps it after a row click.

The rule that outlives this commit: ObservableRoster is the store and the list
a view is bound to is a mirror of it, never the other way round.

docs/PROJECT_CONTEXT.md

@@ -10,12 +10,12 @@ KST4Contest is a Java/JavaFX desktop client for ON4KST chat focused on VHF/UHF/m
 
 ## Current Architecture
 
-- Java 21 / JavaFX desktop application built with Maven.
-- Main code is under `src/main/java/kst4contest/`.
+- Java 21 desktop application built with Gradle in the modules `core` (domain, free of user-interface technology) and `app-desktop` (the JavaFX user interface).
+- Domain, controller and network code is under `core/src/main/java/kst4contest/`, the user interface under `app-desktop/src/main/java/kst4contest/`. Package names are shared across both modules.
 - Responsibilities are separated across controller, service, logic, model, utility and view areas.
 - Network/parser/service/controller/UI boundaries should remain explicit.
 - Long-running network/message processing must tolerate malformed or incomplete external input without terminating processing threads.
-- JavaFX `ObservableList` state is a UI projection, not the canonical worker-thread domain store.
+- `ObservableRoster` state in `core` is the canonical store; the `ObservableList` a `TableView` is bound to is a mirror of it, not the store.
 
 ## Important Invariants
 
@@ -36,19 +36,26 @@ KST4Contest is a Java/JavaFX desktop client for ON4KST chat focused on VHF/UHF/m
 - Features that depend on frequency should use the current/actual QRG according to current implemented rules; do not silently revert to a fixed 144 MHz default.
 - Complete digit-only frequencies use their final three digits as the kHz part and are accepted only when the resulting MHz value lies within a supported `Band` range. The same full-frequency parser is used for station names and public or directed chat messages. Relative QRG rules and bare three-digit context handling remain separate.
 
-### JavaFX/threading
+### Threading
 
 Conceptually:
 
 ```text
-thread-safe canonical domain state
+thread-safe canonical domain state in core
+        |   kst4contest.observe: ObservableRoster / ObservableValue
         |
-        | projection on JavaFX Application Thread
+        | hand-off via UiDispatcher
         v
-JavaFX ObservableList / UI state
+mirror on the user-interface thread (FxRosterBinding -> ObservableList)
 ```
 
-`MessageBusManagementThread` must not directly iterate or mutate UI-bound JavaFX collections. UI-visible changes should cross the controller/UI boundary and run on the JavaFX Application Thread.
+`core` contains no user-interface technology. It observes through `kst4contest.observe` and hands UI-visible work over through `UiDispatcher`, whose JavaFX implementation is `JavaFxUiDispatcher` in `app-desktop`.
+
+`MessageBusManagementThread` must not directly iterate or mutate a UI-bound collection; it works on snapshots. UI-visible changes cross the controller/UI boundary through the dispatcher.
+
+A roster hands out immutable snapshots and has no incremental change protocol: listeners receive the whole new content. `FxRosterBinding` mirrors a roster into the `ObservableList` a `TableView` holds, and must be disposed when its owner goes away — `shutdownRuntime()` releases all of them, which is what keeps a discarded runtime from surviving an operator profile switch.
+
+A batch of changes belongs in one `SimpleRoster.mutate(...)`, not one call per element: with several thousand ON4KST users the difference is not cosmetic.
 
 ## Configuration and Layout Persistence
 
@@ -119,7 +126,7 @@ CR/LF framing, XML framing, ports/transports, callsign normalization and frequen
 - Automatic QRG updates require both an enabled source and valid incoming `RadioInfo` or Win-Test `STATUS` data. Merely enabling a source does not provide or validate a current QRG.
 - UCXLog-compatible QSO packets and Win-Test `ADDQSO` packets are converted into one validated external-QSO state. Logger-specific numeric, metre and centimetre values and Win-Test band IDs are normalised once; the resolved band is then the sole source for per-band Worked and worked-grid state.
 - A missing or unknown logger band sets only the global Worked state. Worked-grid state requires both a recognised project band and a valid locator; no band or locator is inferred. Packets without a usable callsign are discarded without terminating the listener.
-- External logger threads do not read or mutate the JavaFX user-list projection. `ChatController` applies global and per-band Worked state to every active variant of the base callsign on the JavaFX Application Thread before evaluating a band-upgrade notice.
+- External logger threads do not read or mutate the user-list mirror. `ChatController` applies global and per-band Worked state to every active variant of the base callsign on the JavaFX Application Thread before evaluating a band-upgrade notice.
 - The established Win-Test handling for 24, 47 and 76 GHz remains unchanged. Their Worked flags are retained, while only frequencies represented by the project `Band` model can create worked-grid state.
 
 ### Win-Test log recovery
@@ -189,7 +196,7 @@ CR/LF framing, XML framing, ports/transports, callsign normalization and frequen
 
 ## Build / Verification
 
-- Use the repository Maven wrapper (`.\mvnw.cmd` on Windows).
+- Use the repository Gradle wrapper (`.\gradlew.bat` on Windows).
 - The project uses Java 21 / JavaFX 21.x at this context snapshot.
 - JUnit 5/Mockito, PMD and SpotBugs are part of the verification environment.
 - Build/test configuration has historically allowed some test/static-analysis failures not to fail the process exit code. Always read actual summaries/reports.