Devnet up, fixed shielded-pool program ID

Stood up the SDK devnet docs + quickstart guide. The shielded-pool program ID had a stale value in the SDK's default config — three minutes of "why does my proof verify but the CPI fails" before I noticed. Hardcoded constant in crates/zera-sdk-rs/src/constants.rs was pointing at the old program; now derived from ZERA_NETWORK env so devnet/mainnet stay separate by construction. Tip if you're ever…