fix(core): sync Gemini Titan order and websocket handling

[realfishsam]

Summary

• Read Gemini Titan active/history orders from paginated { orders, pagination } envelopes and request limit/offset pages.
• Return the raw canceled order from /order/cancel through the normalizer instead of a placeholder order.
• Update Gemini Titan order book WebSocket subscriptions to the documented @depth@100ms stream and route snapshots by s.
• Add targeted unit tests for the fixed fetcher and websocket paths.

Fixes #430
Fixes #413
Fixes #412
Fixes #419

Test Plan

• npm test --workspace=pmxt-core -- --runTestsByPath test/exchanges/gemini-titan-fetcher.test.ts test/exchanges/gemini-titan-websocket.test.ts --runInBand
• npm run build --workspace=pmxt-core (attempted; tsc was killed by the local runner with exit 137/OOM before completion)
• npm exec --workspace=pmxt-core -- tsc --noEmit --pretty false --skipLibCheck --incremental false (attempted; killed with exit 137/OOM)

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Syncs Gemini Titan authenticated order handling and websocket depth stream handling with the venue response envelopes/stream names.

Blast Radius

Gemini Titan fetcher/index/types/websocket plus targeted fetcher and websocket Jest tests.

Consumer Verification

Before (base branch):
Base expected active/history order shapes and depth stream names that this PR says drifted from Gemini Titan behavior; I did not hit live Gemini authenticated APIs.

After (PR branch):
PR adds paginated order envelope handling, normalizes cancel responses, changes depth stream subscription to {symbol}@depth@100ms, and handles s symbol fields. Core build/Jest passed (26 suites, 649 tests); root verification then hit missing pytest.

Test Results

• Build: PASS
• Unit tests: CORE PASS (26 suites, 649 tests; root verification blocked by missing pytest)
• Server starts: PASS during root verification
• E2E smoke: NOT VERIFIED against live Gemini Titan API/WebSocket

Findings

No blocking findings.

PMXT Pipeline Check

• Field propagation (3-layer): N/A
• OpenAPI sync: N/A
• Financial precision: N/A
• Type safety: OK
• Auth safety: OK — authenticated headers tested without exposing credentials

Semver Impact

patch -- venue integration drift fix.

Risk

Live Gemini Titan API/WebSocket behavior remains unverified; evidence is from targeted mocked tests and build.