Mark price, oracle price, funding rate and OI for one coin

Composite snapshot derived from HL `metaAndAssetCtxs`. Cached 5s — hot endpoint. Unknown coin → 404 `unknown_coin`. Funding rate is the per-hour rate (HL convention) as a decimal (e.g. 0.00001 = 0.001%/hr). Sandbox-aware: when called with a sandbox API key, returns a frozen mock snapshot and stamps `X-Artery-Sandbox: true` on the response.