feat(windows): workmanager_windows — Task Scheduler based background execution (fixes #324)

[ened]

Summary

Adds the Windows desktop port for flutter_workmanager (tracked in #324):
a new federated package workmanager_windows implementing
WorkmanagerPlatform on top of Task Scheduler, plus minimal core
integration and docs.

v1 uses the schtasks CLI only — no COM, no C++, no win32 dependency,
pure Dart (the design doc's "schtasks CLI" option, which keeps the package
fully unit-testable off Windows).

Fixes #324

What's implemented

• One-off tasks — schtasks /Create /SC ONCE at now + initialDelay
  (minute granularity; a zero/sub-minute delay is rounded up to the next
  minute so the task always fires).
• Periodic tasks — schtasks /Create /SC DAILY /RI <minutes>; frequency
  clamped to 1 minute..416 days (the schtasks /RI range).
• inputData persistence — Android-style on-disk payload: a JSON file
  per uniqueName under %LOCALAPPDATA%\workmanager_windows\payloads,
  handed to the headless process via --payload-file <path>.
• cancelByUniqueName / cancelAll — schtasks /End (running
  instance) + schtasks /Delete + payload cleanup; idempotent for missing
  tasks.
• isScheduledByUniqueName / printScheduledTasks — via
  schtasks /Query (CSV parsed and filtered to tasks owned by the plugin,
  prefixed workmanager_).

Headless execution pattern

Mirrors workmanager_web/lib/execution.dart: a small Flutter-free registry
(WorkmanagerExecution) holds the handler registered by the callback
dispatcher, and a runner detects the Task Scheduler launch argument in
main():

void main(List<String> args) {
  if (WorkmanagerWindows.maybeRunBackgroundTask(args, callbackDispatcher)) {
    return;
  }
  runApp(const MyApp());
}

The runner parses --background-task <taskName>, loads the payload JSON,
invokes the registered handler, logs the result and exits with 0/1 (the
exit code becomes Task Scheduler's "Last Task Result"). The dispatcher
registers its handler through WorkmanagerWindows().executeTask(...) (the
registry), not Workmanager().executeTask(...) — the handler runs in a
separate process on Windows.

Unsupported surface (kept honest)

• Constraints (networkType, requiresCharging, requiresDeviceIdle,
  ...) and backoff are accepted but ignored (documented no-ops) —
  schtasks does not expose WakeToRun/idle/battery/AC/network conditions.
• existingWorkPolicy is not honored — re-registering a uniqueName
  always replaces the task (/F, which matches WorkManager's default
  REPLACE policy).
• tag accepted but ignored; cancelByTag throws UnsupportedError.
• iOS-only task types (registerProcessingTask, registerHealthResearchTask,
  registerContinuedProcessingTask) throw UnsupportedError (same as web).
• Per-user tasks only: "run whether the user is logged on or not"
  requires credentials/SYSTEM and therefore admin — documented as an
  opt-in an admin can enable manually; not exposed in the v1 API.
• Locale caveat: schtasks parses dates with the system regional
  format; v1 emits MM/DD/YYYY (en-US assumed).

Test approach

ProcessRunner is injectable (default impl is dart:io Process.run), so all
tests run without Windows:

• test/schtasks_test.dart — command construction: /SC ONCE vs
  /SC DAILY|/RI, /TR quoting with spaces, /Delete, /Query,
  frequency clamping, date/time formatting, CSV parsing.
• test/payload_store_test.dart — JSON payload round-trip, null inputData,
  filename sanitization (path traversal), delete/deleteAll, non-JSON
  rejection.
• test/execution_test.dart — --background-task / --payload-file arg
  parsing and runBackgroundTask exit codes (0 success / 1 failure, handler
  throws, no handler).
• test/workmanager_windows_test.dart — full platform surface against a
  fake process runner (registration args, rollback on schtasks failure,
  cancel/cancelAll, queries, dispatcher wiring, unsupported surface).

Gates run: melos bootstrap ✓, dart analyze (no issues) ✓, dart test
(41 pure-Dart tests) + flutter test (61 total) all pass ✓,
dart format --set-exit-if-changed clean ✓. No flutter build windows run
— the package is pure Dart and needs no Windows toolchain.

Notes for reviewers

• workmanager depends on workmanager_windows via a path dependency
  (the package is not published yet), so dart analyze on workmanager
  reports the expected invalid_dependency warning and the CI
  publishable job will fail on this draft. Once workmanager_windows is
  published, the dependency flips to a caret constraint (melos release
  handles this).
• BackgroundTaskResult migration (feat!: BackgroundTaskResult enum instead of bool (fixes #23) #712): this PR deliberately builds
  against the current Future<bool> handler contract; the desktop packages
  will migrate to the BackgroundTaskResult enum together with feat!: BackgroundTaskResult enum instead of bool (fixes #23) #712.
• Parallel Linux PR: a sibling PR adds workmanager_linux and touches
  the same platform-selection block in workmanager_impl.dart — expect a
  merge conflict there; trivial to resolve in either direction.
• Windows CI: no Windows runner is configured in this repo's workflows
  yet; a windows-latest job for workmanager_windows (analyze + test) is a
  natural follow-up, as is wiring the example app.
• A real-machine smoke test on Windows (register → wait → verify headless
  run → cancel) is recommended before this leaves draft.

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