Add universal caching layer with get_or_set() method

Implements SimpleCache for drivers to minimize redundant SCPI calls
between polling loops and API requests.

Key features:
- Thread-safe caching with RLock
- TTL support with fractional seconds (e.g., 0.6s) or no expiry
- get_or_set() method for concise cache-or-compute pattern
- Hybrid support: direct values and callables
- Lazy expiry and metrics tracking (hits, misses, evictions)

Changes:
- Add cache.py: SimpleCache class with get_or_set() method
- Add test_cache.py: 40 unit tests covering all features
- Add test_owon_oel_caching.py: 11 integration tests
- Migrate owon_oel driver to use get_or_set() (75% code reduction)
- Add CACHE_DESIGN.md: comprehensive documentation
- Update CLAUDE.md: add caching to Key Modules and driver guide

Performance: 3x speedup in owon_oel polling, 10x reduction in SCPI calls

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: MarkoVcode

CLAUDE.md

@@ -359,6 +359,16 @@ BenchMesh stores all user data in `~/.benchmesh/` to ensure configurations, Node
 
 Driver should accept `transport: Transport` in constructor and use it for all communication. The Transport interface supports multiple physical transports (SerialTransport for RS232/USB-Serial, with USB TMC and TCP/IP support planned).
 
+**Optional Caching:**
+Drivers can use `SimpleCache` to minimize redundant SCPI calls between polling and API requests:
+- Import: `from ...cache import SimpleCache`
+- Initialize in `__init__()`: `self.cache = SimpleCache()`
+- Use in `poll_status()`: Check `self.cache.get(key)` before querying, call `self.cache.set(key, value)` after query
+- Invalidate in `set_*` methods: Call `self.cache.invalidate(key)` when device state changes
+- See `drivers/owon_oel/driver.py` for reference implementation
+- See `CACHE_DESIGN.md` for complete caching guide and migration steps
+- Typical performance gain: 3x speedup in polling (owon_oel measured)
+
 ## Key Modules
 
 - `serial_manager.py`: SerialManager orchestrates all device connections and worker threads
@@ -375,6 +385,7 @@ Driver should accept `transport: Transport` in constructor and use it for all co
   - Prevents arbitrary method execution - only `query_*` and `set_*` methods can be called via API
   - Provides `/instruments/{class}/{device_id}/methods` for dynamic method discovery
 - `method_inspector.py`: Python introspection utility for discovering driver methods with parameter/return metadata
+- `cache.py`: SimpleCache universal caching layer for driver values with TTL support and thread safety
 - `connection.py`: DeviceConnection tracks connection state per device
 - `reconnect.py`: ReconnectPolicy implements backoff strategy