feat: add unit test suite

[aratan]

Problem Description

The project has zero tests. Every code change is a blind deployment — no regression coverage, no safety net for concurrency fixes, no validation of edge cases. This is the highest-risk gap in the project.

Proposed Solution

Add a Go test suite covering at minimum:

• StateManager (BlockIP, UnblockIP, manageUnblocks, IsBlocked) — concurrency safety under -race
• MLEngine (AnalyzePacket, feature extraction) — heuristic thresholds
• SteganographyDetector (AnalyzePacket, calculateEntropy) — entropy boundaries
• WhitelistManager (Add, Remove, IsWhitelisted) — concurrent read/write patterns
• ProtocolReconstructionEngine (extractFlowKey, generateSessionID, createNewSession, sessionCleanup) — no data races

The StateManager and WhitelistManager are the highest-value targets due to their concurrent access patterns and the TOCTOU bugs recently fixed.

Affected Area

Code quality / testing

Alternatives Considered

• Integration tests with a real pcap interface (requires root + hardware) — too heavy for CI
• Manual testing only — current state, unsustainable