feat: implement background compaction for Lance fragments (#16)

[jja725]

Summary

Implements #16 - Background compaction for Lance fragments with comprehensive features:

• ✅ Manual compaction via compact() method
• ✅ Optional background compaction with configurable intervals
• ✅ Comprehensive configuration (thresholds, quiet hours, intervals)
• ✅ Advanced observability (stats API, metrics, structured logging)

Changes

Rust Core:

• Added CompactionConfig and CompactionStats types
• Implemented compact(), should_compact(), compaction_stats() methods
• Background compaction task with Tokio interval timer
• Graceful shutdown via Drop implementation
• Added tokio and tracing dependencies

Python API:

• PyO3 bindings for all compaction methods
• High-level API with comprehensive docstrings
• Configuration parameters in Context.create()

Tests:

• 10 comprehensive integration tests (all passing ✅)
• Tests cover manual/background compaction, quiet hours, data integrity

Usage Example

Manual Compaction:
```python
ctx = Context.create("context.lance")
for i in range(100):
ctx.add("user", f"message {i}")

metrics = ctx.compact()
print(f"Removed {metrics['fragments_removed']} fragments")
```

Background Compaction:
```python
ctx = Context.create(
"context.lance",
enable_background_compaction=True,
compaction_interval_secs=300,
compaction_min_fragments=10,
quiet_hours=[(22, 6)], # 10pm-6am
)
```

Check Status:
```python
stats = ctx.compaction_stats()
print(f"Fragments: {stats['total_fragments']}")
print(f"Last compaction: {stats['last_compaction']}")
```

Test Results

```
10 passed in 5.39s
✅ Manual compaction reduces fragments
✅ Data integrity preserved
✅ Concurrent writes work
✅ Compaction stats accurate
✅ Custom options work
✅ Background compaction triggers
✅ Quiet hours respected
✅ Metrics structure correct
✅ Empty context handled
✅ Multiple compactions work
```

Architecture

• Hybrid approach: Both manual and optional background compaction
• Thread-safe: Uses Arc for state management
• Non-blocking: Background task runs in separate Tokio task
• Graceful shutdown: Drop implementation aborts background task
• Lance MVCC: No explicit locking needed, leverages Lance's versioning

Checklist

• Tests added and passing
• Documentation updated
• Code follows project conventions
• All files properly formatted