feat: Add WebSocket order book delta validation tests (#4)

[BERT70293]

Summary

Add comprehensive WebSocket order book delta validation tests and supporting delta infrastructure, satisfying all acceptance criteria in #4.

Changes

market/orderbook/orderbook.go

• Added Delta struct representing a single-side order book level change via WebSocket
• Added ValidateDelta() function — validates symbol, side, price, quantity, and sequence ordering
• Added ApplyDelta() — validates then atomically applies a delta; leaves book unchanged on error
• Added ApplySnapshot() — applies a full depth snapshot with validation
• Added GetSequence() and GetOrdersCount() helpers
• Added upsertLevel() for insert-or-update level semantics
• Added error sentinels: ErrSymbolMismatch, ErrInvalidSide, ErrStaleSequence, ErrInvalidPrice, ErrInvalidQuantity

market/orderbook/orderbook_test.go (new, 650+ lines)

• Malformed payload rejection: negative/zero price, negative quantity, out-of-range side, symbol mismatch
• Sequence validation: stale (equal/lower) sequence rejection, valid increment acceptance
• State preservation: book unchanged after invalid delta (wrong symbol, stale seq, bad price, bad quantity)
• Snapshot integrity: symbol mismatch and negative price snapshots preserve previous state
• Recovery path: valid snapshot → valid deltas → correct final state
• Edge cases: zero-quantity removal, new price level insertion, closed book rejection, sequence tracking
• All tests are deterministic (no network, no random, no time-dependent logic)

market/matching/engine_test.go (new, 300+ lines)

• Order validation: zero/negative quantity, zero/negative limit price, market order acceptance
• Symbol-not-found errors for place/cancel/delta
• Delta validation through engine-owned order books
• State preservation on bad deltas
• Cancel order updates book
• Shorting disabled enforcement
• Trade count and empty trades

market/ws/server_test.go (new, 590+ lines)

• Hub register/unregister/broadcast lifecycle
• Slow consumer disconnect behavior
• Delta JSON message round-trip validation (valid, malformed price string, missing fields, non-numeric sequence)
• HTTP handlers: /health, /depth
• WebSocket upgrade and message echo (via httptest)
• Concurrent access safety: 50 concurrent writers + 50 concurrent readers
• Full integration test: snapshot → delta stream (update, insert, remove) → stale rejection → state preserved → snapshot integrity verified

Testing

• Packages: market/orderbook, market/matching, market/ws
• All tests are self-contained, deterministic, and require no live services
• Cross-platform compatible (Windows/Unix)

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging

[ghost]

Thanks for the PR. I don't see the encrypted build diagnostic log artifact (*.logd / builds.compact) committed on this branch yet. Please rerun the build diagnostic/encryption step with the fixed encryptly tooling and commit the generated encrypted log artifact to this PR (typically under diagnostic/).

Blocking review until the encrypted diagnostic log artifact is present.