[MCA Inspector] Add world save directory support to MinecraftAppFixture

## Summary

Add world save directory property to MinecraftAppFixture so integration tests can read .mca region files directly.

## Tasks

- [ ] Expose WorldSaveDirectory property on fixture
- Captures mount path from Docker container setup
- Used by AnvilRegionReader in tests
- [ ] Create AnvilTestHelper convenience wrapper
- Methods: VerifyStructureAsync(...), VerifyBlockRangeAsync(...)
- Handles save-all flush + delay pattern
- [ ] Document usage: "Call fixture.Rcon.SendCommandAsync('save-all flush') before reading"
- [ ] Add sample code to test helper

## Acceptance Criteria

- [ ] Fixture property accessible in all integration tests
- [ ] AnvilTestHelper compiles and has 3 example usages documented
- [ ] Sample integration test uses both RCON and MCA methods

Start by reading the MinecraftAppFixture Docker container setup and current AnvilRegionReader usage in integration tests. Review the requested AnvilTestHelper methods and the save-all flush guidance, then add the property, helper, documentation, and sample integration test. Done means the fixture is accessible across integration tests, the helper compiles with three documented examples, and the sample uses both RCON and MCA methods.

Written by the indexing model from the issue text.