feat(config): document paired per-network SAC and NFT watch-list env vars (#162)

[kaylachi]

Closes #162

Summary of Changes

Adds documentation and verification for paired per-network SAC and NFT watch-list environment variables (SAC_CONTRACT_IDS_TESTNET, SAC_CONTRACT_IDS_MAINNET, NFT_CONTRACT_IDS_TESTNET, NFT_CONTRACT_IDS_MAINNET), enabling dual-network and single-network deployments to configure distinct watch-lists per chain with backwards-compatible single-variable and per-network default fallbacks.

Key Changes

1. Configuration & Documentation (.env.example):

   • Documented SAC_CONTRACT_IDS_TESTNET and SAC_CONTRACT_IDS_MAINNET with native XLM SAC defaults for testnet and mainnet.
   • Documented NFT_CONTRACT_IDS_TESTNET and NFT_CONTRACT_IDS_MAINNET for explicit CAP-46 NFT tracking per network.

2. Resolution Logic & Fallback Order (src/indexer.ts):

   • resolveSacContractIds(network): Resolves SAC_CONTRACT_IDS_${NETWORK} -> SAC_CONTRACT_IDS -> legacy CONTRACT_IDS -> network default native XLM SAC (DEFAULT_XLM_SAC_MAINNET / DEFAULT_XLM_SAC_TESTNET).
   • resolveNftContractIds(network): Resolves NFT_CONTRACT_IDS_${NETWORK} -> NFT_CONTRACT_IDS -> empty array.

3. Test Suite (src/__tests__/multiNetworkIndexer.test.ts):

   • Validates per-network SAC and NFT watch-list resolution and priority order.

Verification Status

• npm test: All 13 test suites and 124 unit tests passing cleanly.
• PR opened by: kaylachi.
• Commit author: kaylachi <309505611+kaylachi@users.noreply.github.com>.

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