feat(linux): workmanager_linux — systemd-based background execution (fixes #324)

[ened]

Summary

Implements the Linux desktop port for flutter_workmanager (fixes #324) as a
new federated package, workmanager_linux, following the design in
docs/desktop-support.mdx (systemd user units). Pure Dart — no Pigeon, no
native code: scheduling drives systemctl --user / systemd-run --user
through an injectable ProcessRunner, so the whole test suite runs without
systemd.

What's implemented

• One-off tasks — transient units via
  systemd-run --user --unit=workmanager-<hash> --on-active=<delay>
  (--no-block for immediate runs; --collect so finished units don't
  linger).
• Periodic tasks — a .timer/.service pair written to
  ~/.config/systemd/user/: OnUnitActiveSec for the frequency,
  OnStartupSec for the initial delay, Persistent=true for
  WorkManager-style catch-up of runs missed while the system was off.
• Headless execution — units launch the app binary
  (Platform.resolvedExecutable) with --background-task <taskName> --payload <path>. The app's main() calls
  WorkmanagerLinux.maybeRunBackgroundTask(args, callbackDispatcher) which
  runs the dispatcher, invokes the handler and exits 0/1 (failed runs
  show up in the journal).
• Payload persistence — Android-style on-disk inputData: JSON files
  under $XDG_DATA_HOME/workmanager/payloads/, named by a deterministic
  hash of the uniqueName (same hash derives unit names, so no registry is
  needed for query/cancel).
• Full v1 API surface — registerOneOffTask, registerPeriodicTask,
  cancelByUniqueName, cancelAll, isScheduledByUniqueName,
  printScheduledTasks, plus WorkmanagerLinux.executeTask (mirrors
  WorkmanagerWeb.executeTask / workmanager_web's execution.dart
  registry pattern).
• Core integration — Linux branch in workmanager_impl.dart platform
  selection, path dependency in workmanager/pubspec.yaml,
  workmanager_linux added to melos.yaml, docs page docs/linux.mdx +
  sidebar entry + README platform list.

Honest unsupported surface (v1)

• Constraints (network/battery/charging): accepted, ignored. No
  gating in v1.
• Backoff policy: accepted, ignored — failed one-off tasks are not
  retried; failed periodic tasks wait for the next interval.
• existingWorkPolicy: effectively REPLACE; KEEP not implemented.
• Tags: accepted but not tracked — cancelByTag throws
  UnsupportedError (needs a tag registry; cancel by unique name or
  cancelAll).
• iOS-only task types (registerProcessingTask, health research,
  continued processing): UnsupportedError.
• Frequency is honored as-is (no Android-style 15-minute floor).
• Flatpak/Snap sandboxing unsupported (cannot write user units).

Headless dispatcher pattern

The dispatcher registers via WorkmanagerLinux.executeTask (a
WorkmanagerExecution-style registry mirroring workmanager_web), not
Workmanager().executeTask — the latter awaits a platform-channel handshake
that has no native counterpart on Linux and would fail in a headless process.

Testing

Pure Dart unit tests (no systemd): command construction (systemd-run args,
unit file contents, cancel/query commands) against a fake ProcessRunner,
payload round-trip, --background-task arg parsing, and the platform
implementation. melos run test, dart analyze and
dart format --set-exit-if-changed all pass locally (fvm, Flutter 3.44.8).

Note on the runner: the platform-level test file imports
WorkmanagerLinux → workmanager_platform_interface → Flutter, which the
plain VM can't load (dart:ui), so the suite runs under flutter test
(like workmanager_web); the pure command/payload/runner tests also pass
under dart test.

Follow-ups (not in this PR)

• feat!: BackgroundTaskResult enum instead of bool (fixes #23) #712 BackgroundTaskResult migration: this package implements the
  current Future<bool> main API, as do the other desktop/web packages; it
  will migrate to the enum with feat!: BackgroundTaskResult enum instead of bool (fixes #23) #712.
• Parallel Windows PR: a separate PR adds a Platform.isWindows branch
  to the same _ensurePlatformImplementation block — expect a merge
  conflict in workmanager/lib/src/workmanager_impl.dart, that's fine.
• workmanager/test/backward_compatibility_test.dart still expects a
  placeholder (UnimplementedError) on Linux test hosts; once both desktop
  branches land, that expectation should be updated (it passes unchanged on
  macOS/iOS/Android hosts).
• workmanager_linux is a path: dependency of the publishable
  workmanager package for now (pub lint warning invalid_dependency); it
  should switch to ^0.1.0 once the package is released — the melos
  gitignored pubspec_overrides.yaml already wires the path locally.
• CI wiring for dart test/flutter test in workmanager_linux (the
  melos test script already picks it up via flutter_test).

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