fix: dashboard screenshot, referrer-pool tutorial, indexer reorg safety (#80 #85 #86)

[k-deejah]

Summary

Resolves three open issues in a single focused PR.

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Issue closes #85 — Add dashboard screenshot to README

• Added : an SVG illustration that faithfully matches the app's visual design (dark #0b1220 background, #4cc3ff cyan accent, sticky header, tabbed action panel with Create/Pay/Escrow tabs, recipient input rows, and the split-list card grid).
• Inserted the screenshot in README.md immediately after the live-site link, before the prose description, so newcomers see the UI at a glance.

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Issue closes #86 — Tutorial: build a referrer pool with nested splits

• Added docs/tutorial-referrer-pool.md: a full end-to-end walkthrough covering:
  • Why children must be created before parents
  • preview_payout to sanity-check amounts before sending
  • pay to settle a sale in one transaction
  • distribute to flush pool escrow to referrers
  • update_split to swap the referrer roster without touching the marketplace split
  • A plain-text flow diagram tying it all together
  • "What to do next" pointers (deeper trees, pay_many, deposit-then-distribute, locking)
• Cross-linked from the README Docs section and from docs/integrations.md at the existing referrer-pools heading.

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Issue closes #80 — Handle RPC reorgs and make re-scans idempotent

New file: indexer/state.mjs
Pure helpers with zero external dependencies (no @stellar/stellar-sdk):

• validateConfig — moved here from index.mjs so tests can import it without the SDK
• loadState / saveState — state.json now persists cursor and seenIds (sorted array of event ids). Tolerates missing files, corrupt JSON, and legacy files that only have cursor (backwards compatible).
• cursorLedger — decodes the ledger sequence from a cursor string
• isCursorSafeToCommit — returns false when the cursor's ledger is within REORG_DEPTH (default 2, configurable via env) ledgers of the chain tip, holding the cursor back so those ledgers are re-fetched next poll
• deduplicateEvents — skips any event whose id is already in seenIds, making overlapping re-scans fully idempotent

Updated indexer/index.mjs

• Imports all pure helpers from state.mjs
• Poll loop guarded by isMain check so the file can be imported by tests without triggering RPC calls or process.exit
• Cursor only advanced when isCursorSafeToCommit returns true; events within the reorg window are re-fetched and deduplicated automatically

Updated indexer/index.test.mjs

• 20 new tests covering validateConfig, cursorLedger, isCursorSafeToCommit, deduplicateEvents (including a reorg simulation), loadState/saveState round-trips, legacy state compatibility, and large seenIds sets
• All 23 tests (20 new + 3 existing rpc-backoff tests) pass: node --test index.test.mjs rpc-backoff.test.mjs

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CI checklist

• No Rust files changed — cargo fmt, cargo clippy, cargo test unaffected
• No SDK/app TypeScript files changed — npm run build unaffected
• No behaviour changes to contract code — no no_std / typed-error concern
• New indexer behaviour covered by tests per CONTRIBUTING.md

[Spagero763]

This PR's contract check is failing on markdown-link-check / cspell, but not because of anything in your diff — your branch was forked before a docs fix landed on main (the flagged word/link doesn't exist in your changes, and main passes this check cleanly). Since "Update branch" is enabled on this repo, click it on this PR and CI should go green without any code changes on your end.