Define abstract repository interface for metrics

Place the abstract class in the `domain/repositories/` layer (not `data/`), following clean architecture conventions — the domain layer defines the contract, the data layer implements it. Method signatures should use `DateTimeRange` from `dart:ui` or a project-defined equivalent rather than raw `DateTime` start/end pairs, to make the period boundary unambiguous. For `saveMetricSnapshot` and `deleteMetricSnapshot`, use `String snapshotId` as the identifier rather than a full model object for `delete`. For `getMetricSnapshot`, return `Future` (nullable) to indicate that a snapshot may not exist for the requested ID — this is the repository query convention used throughout the project.

When generating the mock with Mockito's `@GenerateMocks`, add `@GenerateMocks([BufdirMetricsRepository])` to the test file and run `dart run build_runner build` — or hand-write the mock using `mocktail`'s `class MockBufdirMetricsRepository extends Mock implements BufdirMetricsRepository {}` pattern, which requires no code generation. This interface is the single most important artifact produced by the data layer epic — its stability directly determines the velocity of all service-layer development and testing that follows.

A single canary unit test file (`test/features/bufdir/domain/bufdir_metrics_repository_test.dart`) must: (1) Instantiate `MockBufdirMetricsRepository` (or the hand-written mock) and verify it compiles without error. (2) Stub one method (e.g., `fetchRawActivities`) and assert the stub returns the configured value — confirming the mock framework integration is working correctly. (3) Verify that calling an unstubbed method on the mock throws a `MissingStubError` (Mockito) or equivalent, confirming strict mock behavior is enforced. This canary test is intentionally minimal — its purpose is to validate that the interface and mock are valid Dart before service-layer tests are written against them.

Coverage target: canary test only; implementation tests belong in later tasks.