Cover indexer reorg final state rewrites

[kitWarse]

Summary

• Added reorg fixtures for an orphaned ledger sequence and its canonical replacement.
• Updated deterministic IDs for market config snapshots and position changes so canonical replay for the same ledger rewrites affected records instead of leaving stale duplicates.
• Added mapping test coverage that replays orphaned events, replays canonical events twice, and asserts the final position, position change, and order state is deterministic.

Issue

• Closes Indexer: ledger reorg handling #336

Changes

• Keyed market config snapshots by market key and ledger.
• Keyed position changes by position key, ledger, and change type.
• Added
  eorged-ledger-sequence.json fixture data.
• Added assertions for stale value replacement and stable final entity counts.

Validation

• Attempted: �un test apps/s03-indexer/tests/mappingHandlers.test.ts
• Result: blocked locally because @stellar/stellar-sdk is not installed in this fresh clone.

Notes

• Full CI/package validation was not run locally per request.
• No dependency or lockfile changes were committed.

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