Compute percent return between start and end values — call when computing percent return between two values

Compute percent return between start and end values — call when computing percent return between two values. Use on caller-supplied ticks/returns when agents need signal prep without a licensed live market feed (ottoai-class demand, first-party transform). Returns deterministic machine-readable JSON for $0.003 USDC via x402 on Base. Execution is first-party, local-only, stateless, and memory-only with no paid upstream, no input retention, and no claim of live chain tip, web search, or media gen…