…e, bug fixes, and comprehensive test coverage

Multi-client infrastructure: ClientState/ClientRegistry for per-client
offloading layers, device inference times, variance detection, and network
speed tracking across all three communication modes. Clients identified by
device_id with backward-compatible keyword-only args for legacy paths.

Bug fixes: WebSocket disconnect handling, NTP unreachable code (3 files),
wrong method name, logger swapped values, WS client missing acquisition_time,
type annotations (6), bare excepts (2), mutable default, syntax fix.

Test suite: 255 tests at 58% coverage including unit tests for ClientState,
MessageData, DelaySimulator, RequestHandler, NTP sync, logger config,
edge delay config, and integration tests for all three multi-client modes.

…e WebSocket framing, harden security, clean up dead code, and add docstrings

- Extract duplicated NTP sync logic into shared NtpSync class with exponential
  backoff, max retries, and broader exception handling (OSError)
- Cache TF model in Edge.run_inference() to avoid reloading from disk per request
- Cache JSON file reads (device/edge inference times, layer sizes) in RequestHandler
- Add thread-safe locking on class-level CSV state in RequestHandler
- Replace WebSocket size-based binary routing with 1-byte type prefix framing
  (0x01=device_input, 0x02=inference_result) with legacy fallback
- Add path validation on /api/set_simulation_csv (must be under data/results/, .csv)
- Replace hardcoded layer index 58 with settings.yaml-driven constant
- Fix broken bare imports in model_split.py and model_build.py
- Remove dead code: init_logger.py, logging.yml, get_offloading_layer_random,
  ModelManager.get_model_layer, OffloadingAlgo.get_info, LOG_INDENTATION_UNIT
- Remove deprecated docker-compose version key
- Add missing __init__.py files for proper Python package structure
- Add Google-style docstrings and type annotations to all public methods
- Update test mocks for NTP path changes; add path validation and WS framing tests

- Untrack src/server/.DS_Store (macOS artifact, 6148 bytes)
- Replace duplicate .DS_Store entries in .gitignore with a single
  recursive glob pattern (**/.DS_Store) to catch all subdirectories
- Remove duplicate *.swp/*.swo entries from OS files section

Closes #14