Upgrade Kotlin from 1.6.21 to 2.x

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## Context

During a dependency update pass (see PR #424 and related work), several dependencies could not be upgraded because they require Kotlin 2.x:

| Dependency | Blocked version | Reason |
|---|---|---|
| `io.mockk:mockk` | 1.14.x | Transitively requires `kotlin-stdlib:2.1.20` |
| `com.github.ben-manes.versions` Gradle plugin | 0.54.0 | Requires Kotlin 2.x compiler |
| `io.gitlab.arturbosch.detekt` | 1.23.x | Uses Kotlin 1.9 compiler API |
| `org.jetbrains.kotlinx:kotlinx-coroutines-core` | 1.10.x | Requires Kotlin 1.8+ |

## What needs to happen

- Upgrade `org.jetbrains.kotlin.jvm` plugin from `1.6.21` to `2.x` (latest stable: `2.1.20`)
- Update `languageVersion` in `KotlinCompile` options (currently `"1.6"`)
- Upgrade `kotlinx-coroutines-core` to `1.10.x`
- Verify `allWarningsAsErrors = true` still passes (new compiler warnings are expected)
- Re-test all modules after the upgrade
- Unblock the deferred dependency upgrades listed above

## Notes

- The Kotlin 2.x migration guide should be consulted: https://kotlinlang.org/docs/k2-compiler-migration-guide.html
- Android example module (`examples/android-chatter`) may need separate attention given its AGP version constraint

Start with the org.jetbrains.kotlin.jvm plugin version and KotlinCompile languageVersion configuration, then consult the Kotlin 2.x migration guide. Run tests across all modules, including examples/android-chatter, and verify allWarningsAsErrors passes while the listed dependency upgrades are unblocked.

Written by the indexing model from the issue text.