refactor(scheduler)!: typed TaskId, LocalTime, @Serializable inputData

[kdroidFilter]

Summary

Three breaking refactors of the public scheduler API for stronger typing.

• TaskId value class — @JvmInline value class TaskId(val value: String) with regex validation in init. Replaces String taskId across TaskRequest, TaskRegistry, TaskContext, TaskInfo, PlatformScheduler, all 4 backends (Linux/macOS/Windows/Noop), TaskMetadataStore, TaskWrapperScript, DesktopBootReceiver, and scheduler-testing. Raw strings are unwrapped only at JNI/filesystem boundaries; the boot-receiver rejects malformed CLI IDs before any lookup.
• CronExpression takes LocalTime — removed all (hour: Int, minute: Int) overloads. Now everyDayAt(LocalTime), everyWeekdayAt(LocalTime), everyWeekdayAt(DayOfWeek, LocalTime), everyMondayAt(LocalTime), everyHour(). Bounds checks come from LocalTime itself.
• @Serializable inputData — dropped string-keyed TaskData.Builder.putString/putInt/.... TaskData is now an opaque wrapper around a JSON String?. New API: TaskRequest.Builder.inputData(value) reified inline (and an explicit-serializer overload), context.inputData<T>() to decode. TaskMetadataStore persists the payload in a single reserved _inputDataJson key.

kotlinx-serialization-json is added as api to scheduler so consumers don't have to wire it themselves; the kotlinx serialization plugin is enabled in scheduler, scheduler-testing, and scheduler-demo. Demo (SampleTasks, SchedulerDemoView) and tests rewritten against the new API. docs/runtime/scheduler.md rewritten to match.

Breaking changes

• Consumers passing String IDs to enqueue/cancel/isScheduled/getTaskInfo, registering with register("foo") { ... }, or building requests with TaskRequest.periodic("foo", ...) must wrap in TaskId(...).
• Calls like CronExpression.everyDayAt(9) / everyWeekdayAt(18) must switch to LocalTime.of(...).
• Anything reading context.inputData.getString("k") / writing inputData { putString("k", "v") } must define a @Serializable data class and switch to inputData(value) / context.inputData<T>().
• Persistence format changed: pre-existing <taskId>.properties files with arbitrary keys are ignored on next run (only _inputDataJson and the underscore-prefixed bookkeeping keys are read). No migration shim — assumed acceptable for an unreleased module.

Test plan

• ./gradlew :scheduler:compileKotlin :scheduler:compileTestKotlin :scheduler-testing:compileTestKotlin :scheduler-demo:compileKotlin — green
• ./gradlew :scheduler:test :scheduler-testing:test — green
• Manual smoke test: enqueue a periodic task in scheduler-demo on Linux/Windows/macOS, verify it fires and context.inputData<BackupInput>() decodes the payload
• Verify mkdocs serve renders the updated scheduler.md cleanly