refactor(crypto): split aead.rs into focused modules (REFACTOR-001, 8 SP)

Split monolithic 1,529 LOC aead.rs into 4 focused modules for improved
maintainability, testability, and code organization.

New module structure (aead/):
- cipher.rs (488 LOC): Core AEAD types
  - Nonce: 24-byte XChaCha20 nonce with counter and random modes
  - Tag: 16-byte Poly1305 authentication tag
  - AeadKey: 32-byte key with encrypt/decrypt/in-place operations
  - AeadCipher: Legacy API wrapper
  - Key commitment for key-committing AEAD
  - 9 cipher tests

- replay.rs (264 LOC): Replay protection
  - ReplayProtection: 256-bit sliding window
  - Constant-time comparison via subtle crate
  - Multi-word bit shifting for window advancement
  - 7 replay tests

- session.rs (457 LOC): Session management
  - SessionCrypto: Bidirectional encrypted communication
  - BufferPool: Zero-allocation buffer reuse
  - Key commitment in AAD for key-committing AEAD
  - Automatic nonce counter management
  - 7 session tests

- mod.rs (42 LOC): Public re-exports
  - All types re-exported for backward compatibility
  - Module documentation

Benefits:
- Single responsibility per module
- Easier testing and maintenance
- Clearer dependency boundaries
- Reduced cognitive load (4 small files vs 1 large)
- 278 LOC reduction (-18%) through deduplication

All 23 AEAD tests preserved and passing.
No API changes - full backward compatibility maintained.

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

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

Author: doublegate

CHANGELOG.md

@@ -28,10 +28,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **BLAKE3 SIMD (PERF-001, 8 SP)**: rayon + neon features
   - 2-4x faster parallel hashing, ARM64 optimization
 
+### Refactored
+- **AEAD Module Split (REFACTOR-001, 8 SP)**: Improved code organization
+  - Split 1,529 LOC `aead.rs` into 4 focused modules (1,251 LOC total)
+  - `aead/cipher.rs` (488 LOC) - Nonce, Tag, AeadKey, AeadCipher
+  - `aead/replay.rs` (264 LOC) - ReplayProtection sliding window
+  - `aead/session.rs` (457 LOC) - SessionCrypto, BufferPool
+  - `aead/mod.rs` (42 LOC) - Re-exports for backward compatibility
+  - All 23 AEAD tests preserved and passing
+
 ### Documentation
 - Refactoring analysis (18 priorities, complexity metrics)
 
-**Story Points: 44 SP**
+**Story Points: 52 SP**
 
 ## [0.7.0] - 2025-12-01