feat: PlainStateProvider returns key-value pairs with version tracking

[deepld]

Summary

Enhance PlainStateProvider to return version information alongside key-value pairs, enabling downstream consumers to track state changes and implement version-aware caching strategies.

Key API Change:

// Before
pub fn plain_value(&self, plain_key: &[u8], hint: Option<BucketId>) 
    -> Result<Option<Vec<u8>>, S::Error>

// After  
pub fn plain_value(&self, plain_key: &[u8], hint: Option<BucketId>)
    -> Result<Option<(SaltValue, SaltVersion)>, S::Error>

Changes

• New type: SaltVersion (u64) for tracking state value versions
• Trait extension: Added StateReader::value_with_version() with backward-compatible default implementation
• API enhancement: PlainStateProvider::plain_value() now returns (SaltValue, SaltVersion) tuple
• Refactoring: Generalized internal search functions to eliminate code duplication
• Optimization: Added #[repr(align(8))] to SaltValue for improved memory alignment

Backward Compatibility

✅ Existing StateReader implementations continue to work without modification - the default value_with_version() implementation returns version 0

⚠️ Breaking Change: Callers of PlainStateProvider::plain_value() must update to handle the new return type