Soroban event reference doc

[githoboman]

Summary
Closes #144
Created docs/event-reference.md — a reference of every event the indexer recognizes, grounded in the actual code rather than the task title.

What it documents, one section per event, each with a real worked example:

transfer — from/to/amount (from topics[1], topics[2], value)
mint — to/amount; from is null, admin in topics[1] is intentionally ignored
burn — from/amount; to is null
clawback — same shape as burn
Each section has: the topic/value shape table, how the fields map into a TransferRecord, the base64 XDR (taken verbatim from the test fixtures), and the decoded form. The example XDR and decoded values are verified — I ran them through @stellar/stellar-sdk's scValToNative/Address to confirm the exact addresses and amounts, then removed the temp script.

One correction worth flagging: the task title lists transfer, mint, burn, approve, but the indexer actually recognizes transfer, mint, burn, clawback (src/decoder.ts:7 — KNOWN_EVENT_TYPES). There is no approve handling anywhere. I documented the four real events and added a dedicated "A note on approve" section explaining why it's not indexed (it sets an allowance, not a balance movement; its value is a struct, not an i128) and what it would take to add it.

Gitignore change: per your choice, I removed the docs/ line from .gitignore so the doc is trackable. Side effect: the existing docs/cookbook/ directory is now also no longer ignored — worth a glance before you commit if it contains anything you'd rather keep untracked.

Want me to commit these changes (on this Soroban-event-reference-doc branch), or open a PR?

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