Add Chainlink and Spectra Finance to ecosystem.md #151

#### Chainlink
Data Feeds and Data Streams live on Stellar (October 2026).
- **Docs**: https://docs.chain.link/data-feeds/stellar
- **Feed IDs**: https://docs.chain.link/data-feeds/price-feeds/addresses?network=stellar (select Stellar)
- **Proxy (mainnet)**: `CDQBHEUGLDHMSUHG4IH33RJBUTMNCZOT4JIAQR4WZFRDZW6DR6S32ZLZ`
- **Proxy (testnet)**: `CBUF6IADAWPWIDWRTHJNINKXHG2UTIQ6TU2F2HRAXWAI7OT3KJPKK6O4`
- **Interface**: not SEP-40. One proxy serves every feed; select a feed by its 32-byte `data_id`. `latest_round(data_id, decimals)` and `get_round(data_id, round_id, decimals)` return `Round { round_id, answer: I256, timestamp }`. Answers are stored at 18 decimals; pass `18` unless Chainlink approved a lower precision. Interface source: https://github.com/smartcontractkit/chainlink-stellar
- **Example**: BTC/USD `data_id` `01a0b4d920000332000000000000000000000000000000000000000000000000` (same on testnet and mainnet)
- **Data Streams**: pull model; verify signed reports on-chain with the Chainlink Verifier contract. Needs an allowlisted account. Guide: https://docs.chain.link/data-streams/tutorials/stellar-onchain-report-verification

#### Spectra Finance
Fixed-rate yield markets: splits a yield-bearing asset into a Principal Token (PT) and a Yield Token (YT). Live on Stellar mainnet; Stellar markets accept XLM and USDC and swap them into the market asset first.
- **Use Case**: Lock a fixed rate (buy PT), trade future yield (YT)
- **App**: https://app.spectra.finance/fixed-rate?network=stellar
- **Docs**: https://docs.spectra.finance/docs/stellar
- **Developer docs**: https://dev.spectra.finance/docs/stellar (Core v2 integration, router and limit orders, IBT wrappers, PT bridge, oracle)
- **Mainnet contracts**: https://dev.spectra.finance/docs/technical-reference/deployed-contracts#stellar-mainnet (Registry, PT Factory, Router, Limit Order Engine, PT Bridge, PT Oracle Factory)
- **Features**: Core v2 on Soroban, PT/YT limit orders, Blend wrapper (Blend positions as vault shares), PT bridge to EVM, deterministic zero-coupon-bond PT oracle
- **Caveat**: PT market deployment is permissionless. Registry membership is discovery data, not an allowlist.