fix: type GeminiRawEvent.series as object|null

[realfishsam]

Fixes #439

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Changes the type of GeminiRawEvent.series from string to Record<string, any> | null to match the actual Gemini Titan API response shape where series is an object (or null), not a string.

Blast Radius

• Core layer only: core/src/exchanges/gemini-titan/types.ts (raw type definition)
• No normalizer or downstream code references series -- grep confirms it is only defined in this types file and not read by the normalizer

Findings

No blocking findings. The change correctly widens the type to match observed API behavior.

PMXT Pipeline Check

• Field propagation: N/A (raw interface only, not mapped to unified type)
• OpenAPI sync: OK (no BaseExchange.ts change)
• Type safety: OK

Semver Impact

patch -- raw type correction, no behavioral change

Risk

• If series can also be a plain string in some API responses (e.g., older events), the type should be Record<string, any> | string | null to avoid regressions. Verify against live responses to ensure string is truly never returned.