Documented event catalog

[nanaf6203-bit]

Problem Statement
The contract emits 14 events. There's no doc that summarizes the
topic+payload shape of each. Indexers must read source to discover the
formats.

Why it matters
External integrators can implement indexers without studying Rust.

Acceptance Criteria

• New docs/events_catalog.md listing every EVENT_* constant with:
  topic shape, payload shape, when emitted.

Files/Modules Likely Affected

• docs/events_catalog.md (new)

Difficulty Easy. Estimated Effort S.

Labels
area:documentation, priority:medium

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I. Developer Experience & Tooling

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Imported from docs/open-issues-draft.md as draft issue #48 (commit 7becd4b). Original draft-order cross-references in this body have been remapped to current GitHub issue numbers at import time.

[GBOYEE]

Hi team — I'd like to take this on as part of the GrantFox campaign. I'll write the documentation and open a PR referencing this issue shortly.

[OlaGreat]

I'd like to take this on. I'm a software engineer with experience in blockchain, backend systems, and open-source contribution. I've reviewed the issue and I'm confident I can deliver a clean implementation with tests covering the acceptance criteria. Happy to follow the project's existing conventions.

[grantfox-oss]

🦊 GrantFox — @OlaGreat has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #108)
2. Your PR will be reviewed by the StellarTips maintainers

Good luck! Track your progress on GrantFox.