[M1.2.5] Define FlatBuffers serialization layer

[LyeZinho]

Summary

Implement FlatBuffers-based serialization layer for MiMi message protocol with version compatibility checks, data integrity verification, and comprehensive round-trip testing.

Changes

Serialization Module (crates/mimi-core/src/serialization.rs)

• MessageSerializer struct with encode/decode functions
  • encode_with_version(): Binary serialization with version header + checksum
  • decode_with_version(): Deserialization with integrity validation
• Version compatibility: Validates protocol version (min/max bounds)
• Checksum: Fletcher-32 for data corruption detection
• Error handling: Comprehensive error enum (VersionMismatch, MessageTooLarge, InvalidFormat, CorruptedData, etc.)

FlatBuffers Schema (proto/schema.fbs)

• Enumerations (9 types): IntentType, Priority, ComplexityEstimate, ResponseStatus, CacheLayer, MessageBodyType
• Message types (10+ types):
  • Beatrice: UserInput, IntentClassified
  • Mimi: TaskExecution, ExecutionResult
  • Pandora: MemoryUpdate
  • Liliana: LilianaResponse, MoodState, ToneProfile
  • Odlaguna: OdalgunaGate
  • System: SkillPublish, ErrorReport, EventNotification, ControlMessage
• Trace context: Distributed tracing (correlation_id, parent_id, span_id, trace_id)
• Unified Message envelope: Type-safe union of all message bodies with routing metadata

Tests (8 comprehensive unit tests)

✅ Version validation (boundary checking)
✅ Encode/decode roundtrip (payload reconstruction)
✅ Special characters & emoji support (UTF-8 handling)
✅ Checksum detection (data corruption verification)
✅ Message size limits (overflow prevention)
✅ Large payload handling (1MB+ stress test)
✅ Version mismatch detection (compatibility enforcement)
✅ Empty payload handling (edge case)

Verification

• ✅ All 11 library tests pass (8 serialization + 3 existing)
• ✅ cargo fmt clean (rustfmt validation)
• ✅ cargo clippy -D warnings clean (lint checks)
• ✅ No regressions in existing code

Next Steps

• M1.2.6: Implement message routing middleware (topic hierarchy, flow debugging)
• Integration with Zenoh client (M1.2.3) for pub/sub wire protocol