fix: correct native XLM SAC defaults and add derivation-based test

[Daveeee-tech]

closes #222

Summary

Fixes the hard-coded native XLM Stellar Asset Contract (SAC) addresses, which were each shifted one network along. Adds a test that checks them against the SDK's own derivation so this can't regress silently.

Problem

DEFAULT_XLM_SAC_MAINNET and DEFAULT_XLM_SAC_TESTNET in src/indexer.ts, and KNOWN_SAC_IDS in src/indexer/sac-detect.ts, did not match Asset.native().contractId(...):

Network	Previously held	Correct value (SDK derivation)
Mainnet	testnet SAC (CDLZFC3S…)	CAS3J7GYLGXMF6TDJBBYYSE3HQ6BBSMLNUQ34T6TZMYMW2EVH34XOWMA
Testnet	standalone SAC (CDMLFMKM…)	CDLZFC3SYJYDZT7K67VZ75HPJVIEUVNIXF47ZG2FB2RMQQVU2HHGCYSC

All three are valid StrKey contract IDs, so nothing flagged the error. Only the derivation was wrong.

Impact

• Mainnet: with SAC_CONTRACT_IDS unset, resolveSacContractIds() falls back to a contract that doesn't exist on mainnet. getEvents returns nothing forever, and the service looks healthy while the table stays empty.
• Testnet: the fallback points at the standalone SAC, with the same silent-empty failure. Working deployments only avoided this by setting SAC_CONTRACT_IDS explicitly.
• sac-detect.ts: real mainnet XLM transfers missed KNOWN_SAC_IDS and took the slow RPC path. Testnet XLM hit the set through the entry labelled "mainnet", so it was right for the wrong reason.

Changes

• src/indexer.ts: corrected DEFAULT_XLM_SAC_MAINNET and DEFAULT_XLM_SAC_TESTNET.
• src/indexer/sac-detect.ts: corrected the two values in KNOWN_SAC_IDS.
• .env.example:
  • corrected both native SAC fallbacks and the testnet / mainnet fallback comment
  • corrected the "Known mainnet SAC addresses" block and the SAC_CONTRACT_IDS_TESTNET / SAC_CONTRACT_IDS_MAINNET examples
  • corrected the mainnet USDC example to CCW67TSZV3SSS2HXMBQ5JFGCKJNXKZM7UQUWUZPUTHXSTZLEO7SJMI75, which is what new Asset('USDC', 'GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN').contractId(Networks.PUBLIC) produces (the issuer key was already correct)
• Tests: added a test asserting each constant against Asset.native().contractId(Networks.PUBLIC | Networks.TESTNET) instead of against a literal.

it("matches the SDK's own derivation", () => {
  expect(DEFAULT_XLM_SAC_MAINNET).toBe(Asset.native().contractId(Networks.PUBLIC));
  expect(DEFAULT_XLM_SAC_TESTNET).toBe(Asset.native().contractId(Networks.TESTNET));
});

Background

The wrong value originated in #166, which quoted CDLZFC3SY… as the mainnet SAC and carried into docs/DEPLOY_MAINNET.md via #211. The doc was corrected on merge, but the code was not.

Acceptance criteria

• DEFAULT_XLM_SAC_MAINNET / DEFAULT_XLM_SAC_TESTNET equal the SDK derivation for Networks.PUBLIC / Networks.TESTNET
• KNOWN_SAC_IDS holds the same two corrected values
• .env.example corrected (both native fallbacks, known-mainnet-SAC block, USDC example)
• Test asserts each constant against Asset.native().contractId(...)

Notes for reviewers

• Deployments that already set SAC_CONTRACT_IDS explicitly are unaffected.
• Deployments relying on the old defaults were indexing nothing, so after upgrading they will start picking up native XLM events from the network they actually target.

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