Add WebSocket order book delta validation tests

[natemiller23]

Summary

Adds deterministic WebSocket/order book delta validation coverage for bounty #4.

/claim #4

Changes

• Added orderbook.Delta with explicit validation errors for malformed symbols, prices, quantities, stale/out-of-order sequences, and checksum mismatches.
• Applies deltas atomically by validating into cloned book state first, so invalid deltas preserve the existing book.
• Added snapshot replacement plus valid snapshot -> valid delta coverage.
• Added WebSocket delta message parsing tests for malformed payloads and unsupported event types.
• Added matching package coverage proving invalid deltas do not mutate an engine-owned book.

Testing

$go test ./orderbook ./matching ./ws
ok   github.com/tent-of-trials/market/orderbook  0.024s
ok   github.com/tent-of-trials/market/matching   0.023s
ok   github.com/tent-of-trials/market/ws         0.017s

python build.py was attempted under UTF-8, but this Windows workstation is missing required repo toolchains (npm, Rust, gcc, cmake, Java, Ruby, Lua, GHC). The script stops at npm install before producing a new diagnostic artifact. Existing diagnostic/build-00000000.{json,logd} artifacts are unchanged.

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

---

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

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