Upgrade Gradle to 9.6.1, conform card-game to composite-build standard, prep for Gradle 10

[lagergren]

This PR does three things.

1. Upgrade Gradle to 9.6.1

Upgrades the composite build's wrapper from 9.5.0 to 9.6.1. Wrapper artifacts
(gradle-wrapper.properties, gradlew, gradlew.bat, jar) were regenerated by
running the wrapper task twice under the target Gradle, matching the canonical
wrapper form used by the xvm repo. Verified ./gradlew --version reports 9.6.1
and the whole build configures cleanly.

(The second wrapper that used to live under CardGame/ — stuck at 9.4.0 — is
removed entirely rather than upgraded; see below.)

2. Make card-game conform to the composite-build standard

CardGame was self-merged (#12) as a standalone build dropped into the repo
root: its own Gradle wrapper, settings.gradle.kts, gradle.properties and
version catalog, PascalCase-named, and never actually wired into the root
composite build. It ignored the conventions every other example follows.

Brought it in line with the other single-module examples (welcome / banking
/ counter):

• Renamed CardGame → card-game (kebab-case, matching chess-game).
• Deleted the redundant standalone infra: gradlew, gradlew.bat,
  gradle/wrapper/*, settings.gradle.kts, gradle.properties and
  gradle/libs.versions.toml. It now shares the root wrapper, root
  gradle.properties and the root version catalog. The repo again has exactly
  one Gradle wrapper (at the root), as a composite build should.
• Reuses build-logic: build.gradle.kts applies id("xtc-conventions")
  instead of the raw alias(libs.plugins.xtc) + manual xdkDistribution.
• Wired into the root build: include(":card-game") in settings.gradle.kts
  and xtcModule(projects.cardGame) in the root build.gradle.kts — a
  subproject with a type-safe project accessor, like the other single-module
  examples (not an included build like the multi-module chess-game).
• Docker/CI: Dockerfile copies the card-game build file + sources; the
  CardGame/ exclusion is dropped from .dockerignore.

The compiled module is still named cardGame (source unchanged).

3. Prepare build scripts for Gradle 10

Gradle 9.6.1 deprecates the Kotlin DSL delegated accessors (val x by tasks.registering/existing), scheduled for removal in Gradle 10. Switched the root build.gradle.kts and the webapp-conventions build-logic plugin to the named/register APIs (tasks.register<T>("x"), tasks.named(...)) — matching xtclang/xvm#494.

Verification

• ./gradlew :card-game:build — compiles cardGame.xtc and its xunit tests
  pass (through the shared xtc-conventions plugin, with failOnTestFailure=true).
• ./gradlew installDist — succeeds; cardGame.xtc is aggregated into
  build/install/examples/lib/ alongside the other example modules.
• Exactly one Gradle wrapper remains in the tree (the root), at 9.6.1.
• ./gradlew build installDist --warning-mode all — BUILD SUCCESSFUL with no Gradle 10 deprecation warnings; no delegated accessors remain.