Add comprehensive Merkle tree-based anti-entropy mechanism for distributed cache synchronization:

- Implement BuildMerkleTree() to create hash trees from cache data with configurable chunk sizes
- Add SyncWith() method for comparing local/remote trees and pulling newer versions
- Extend DistTransport interface with FetchMerkle() for remote tree retrieval
- Add /internal/merkle HTTP endpoint for tree access over network
- Include merkle sync metrics (operations count and keys pulled)
- Add comprehensive test coverage for sync convergence scenarios
- Support both in-process and HTTP transports (HTTP fetch merkle marked as unsupported)

This enables efficient detection and repair of data inconsistencies between distributed cache nodes by comparing compact tree representations rather than full data sets.

- Add FetchMerkle() and ListKeys() methods to HTTP transport
- Implement periodic auto-sync with configurable intervals and peer limits
- Add /internal/keys endpoint for key enumeration
- Refactor sync logic into modular helper methods
- Add comprehensive HTTP Merkle sync test coverage
- Enhance distributed metrics with auto-sync tracking
- Clean up HTTP transport code and remove redundant comments

This enables full anti-entropy synchronization over HTTP transport
and provides automatic background sync capabilities for distributed
cache consistency.

…semantics

- Add Merkle tree synchronization with timing metrics (build, diff, fetch durations)
- Implement tombstone versioning to prevent key resurrection during anti-entropy
- Add new HTTP endpoints for Merkle tree inspection (/internal/merkle, /internal/keys)
- Introduce configuration options for Merkle chunk size, auto-sync, and key enumeration caps
- Enhance delete operations with versioned tombstones to maintain consistency
- Add comprehensive test suite for Merkle sync edge cases (empty trees, no-diff, single missing keys, tombstone preservation)
- Update documentation with new distributed memory capabilities and configuration options

This enables robust distributed consistency by preventing stale data resurrection
and providing efficient anti-entropy synchronization between cache nodes.

…ogress table

- Add comprehensive tombstone versioning and anti-resurrection guard details
- Document Merkle phase timing metrics and anti-entropy pull counters
- Include roadmap progress table showing current implementation status
- Expand descriptions of delete semantics and remote sync behavior
- Clarify DebugInject tombstone clearing functionality for testing

This update provides much clearer documentation for users and contributors
about the current state of distributed cache features and deletion handling.

- Add configurable tombstone TTL and periodic compaction to reclaim memory
- Implement WithDistTombstoneTTL and WithDistTombstoneSweep options
- Add tombstone metrics tracking (TombstonesActive, TombstonesPurged)
- Enhance quorum reads with targeted stale owner repair
- Refactor consistency logic with collectQuorum helper method
- Update README with new configuration options and metrics
- Add comprehensive test coverage for stale quorum scenarios
- Improve error handling and code formatting in existing tests

This addresses memory management concerns with tombstone accumulation
while improving distributed consistency guarantees through better
read repair mechanisms.

…stale tracking

- Fix variable capture in goroutines for Go <1.22 compatibility
- Add owner tracking to parallel fetch results to enable targeted repairs
- Implement stale owner detection during parallel consensus building
- Add targeted repair mechanism before full replica repair
- Improve code structure and comments for better maintainability

This ensures parallel quorum reads correctly identify and repair stale
replicas while maintaining compatibility with older Go versions that
require explicit variable capture in goroutine closures.