feat: add getChainlinkFeed address lookup

[DhairyaSethi]

Follow-up to #12.

Chainlink demoted several live feeds to the secondaryProxyAddress of an SVR entry. The address keeps answering on-chain, but it is no longer any feed's proxyAddress, so a consumer indexing the generated table by proxy alone stops resolving it. Seven Base feeds are in that state today (ETH, BTC, XRP, SOL, DOGE, ADA, LTC).

getChainlinkFeed({ chainId, address }) indexes both address fields, per chain, and returns matchedOn: "proxy" | "secondary":

getChainlinkFeed({ chainId: 8453, address: "0x71041dddad3595F9CEd3DcCFBe3D1F4b0a16Bb70" })
// { proxyAddress: "0x50015f8b…", secondaryProxyAddress: "0x71041ddd…",
//   name: "SVR ETH / USD", decimals: 8, matchedOn: "secondary" }

The SVR secondary is a different contract with different update behavior from the primary, so matchedOn keeps a caller from silently using one for the other. Unknown address or chain returns undefined, since asking whether an arbitrary address is a feed is a normal question with a legitimate no.

Details: the index is built lazily per chain and cached. A proxy match always wins over a secondary match, though the current table has no address serving both roles across all 17 chains. Name lookup is deliberately out of scope, since matching "ETH / USD" against SVR ETH / USD needs a prefix rule that would hand callers an SVR contract they did not ask for.

Follows the ecosystem/explorers.ts pattern of a hand-written module over generated data. The generator and the generated file are untouched.

vitest run passes (109 passed, 2 skipped, 5 new). Typecheck errors unchanged from main (26, all in seatbelt specs).