fix(gemini-titan): use millisecond nonce for WebSocket authentication

[VaggelisGian]

Fixes #1933

What was broken

buildWsHeaders() in core/src/exchanges/gemini-titan/auth.ts computed the WebSocket handshake nonce as Math.floor(Date.now() / 1000), sending seconds where Gemini's WebSocket API requires a millisecond Unix timestamp. The signed payload carried the same seconds value, so authenticated handshakes landed outside the server's replay-prevention window and were rejected, breaking watchOrderBook/watchTrades before any subscription could start.

The REST path was already correct: fetcher.ts signs payloads with GeminiAuth.nonce(), which returns milliseconds. Only buildWsHeaders() computed a nonce inline with the wrong unit.

What changed

• Removed the /1000 division so the handshake nonce is Date.now() directly.
• Updated the doc comment above it that still said seconds since epoch.
• Added core/test/exchanges/gemini-titan-auth.test.ts, which freezes Date.now() and asserts both the X-GEMINI-NONCE header and the base64-decoded X-GEMINI-PAYLOAD carry the frozen millisecond value, plus an HMAC cross-check of X-GEMINI-SIGNATURE.

Verification

• New tests fail on main (received 1755878400 vs expected 1755878400000) and pass with the patch.
• npx jest -c jest.config.js test/exchanges from core/: 11 suites, 34 tests, all passing.
• Full core jest: 823 passed; the only failure is the pre-existing pmxt-ensure-server sandbox case.
• bash scripts/verify-all.sh on this branch matches a clean checkout of main exactly: the only failures are the three known Windows-only HOME-vs-USERPROFILE sandbox defects (core test/server/pmxt-ensure-server.test.ts plus two sdks/python/tests/test_server_manager.py cases), which pass on ubuntu-latest CI.