fix(indexer): disambiguate dual-shape parsing by arity and enum-tag heuristic (#170)

[trenysx]

Summary

Closes #170

Problem: The parseCampaignRegistered and parseFarmerRegistered parsers used a fragile ypeof === 'string' check to distinguish between the direct-call payload (which includes a real title/name) and the activity-log mirror payload (which emits an ActivityAction enum tag in the same slot). The stellar-sdk can decode fieldless enums as a bare string, a single-element array (['Variant']), or a { tag: 'Variant' } object. A bare-string enum decode would pass the ypeof === 'string' check and corrupt the campaign title/farmer name with the enum variant name (e.g., "CampaignRegistered").

Solution: Disambiguate deterministically by payload structure:

• FarmerRegistered: arity differs — direct call has 4 elements (farmer, name, timestamp, ledger_sequence), activity mirror has 3 (actor, timestamp, ledger_sequence). For 4-element payloads, additionally check if the second element is an enum tag (via isTagLike + PascalCase heuristic).
• CampaignRegistered: both shapes have 4 elements. Use isTagLike for array/object enum shapes, and a PascalCase heuristic (/^[A-Z][a-zA-Z0-9]*$/) for bare strings — enum variants are PascalCase without spaces; user titles typically contain spaces/lowercase.

Added looksLikeEnumTag helper for bare-string enum detection.

Tests added (regression coverage for all three documented enum-decoding shapes):

• FarmerRegistered: bare string, array, object enum tags → no name corruption
• CampaignRegistered: bare string, array, object enum tags → no title corruption
• All existing tests still pass (direct-call titles/names still work correctly)

All 51 unit tests pass; sc --noEmit and eslint clean.