feat(platform): replace InMemoryConfigValueProvider with JavaPreferencesConfigValueProvider on JVM

[kirich1409]

Context

featured-platform/src/jvmMain/.../DefaultLocalProvider.kt contains:

// TODO(#66): Replace with JavaPreferencesConfigValueProvider once available.
public actual fun defaultLocalProvider(): LocalConfigValueProvider = InMemoryConfigValueProvider()

The providers/javaprefs module already implements JavaPreferencesConfigValueProvider with tests. The defaultLocalProvider() fallback on JVM should use it so integrators get persistent local storage automatically.

Acceptance criteria

• featured-platform jvmMain: defaultLocalProvider() returns JavaPreferencesConfigValueProvider instead of InMemoryConfigValueProvider
• Add providers:javaprefs as a dependency of featured-platform for the JVM target only
• Remove the TODO(#66) comment
• Update KDoc on defaultLocalProvider() to reflect the persistent behavior
• Existing tests in providers/javaprefs pass; add a test in featured-platform if needed

Notes

• JavaPreferencesConfigValueProvider uses java.util.prefs.Preferences — verify it is already available on all targeted JVM versions
• Do NOT change Android or iOS actual implementations