test: add file sync integration test suite (cases 1-8) ...

Covers all 8 real-world file sync scenarios:
  Cases 1-4 (small files):
  1. Write from node-1 → node-2 + server mirror receive it
  2. Write from server (via node-1) → all client nodes receive it
  3. Delete from server → all client nodes purged
  4. Delete from node-2 → server mirror + node-1 purged

  Cases 5-8 (20 MB large files, multi-chunk):
  5-8: Same cases as above but with 20 MB files to exercise multi-chunk sync

  Design:
  - Synchronous httpx client (no asyncio overhead)
  - Shared module-scoped swarm_control session with both nodes attached
  - _poll_until() helper for timeout-based convergence checks
  - Auto-skip via requires_nodes marker when Hub/nodes are unreachable
  - Env vars: SYNC_TEST_BASE_URL, SYNC_TEST_NODE1, SYNC_TEST_NODE2