feat: add Stellar announcement range scans

[kaycke1337]

Summary

• Extend Stellar fetchAnnouncements with ledger/timestamp bounds, cursor resume, and a typed RetentionExceededError
• Preserve legacy array return when called without range options while returning { announcements, nextCursor } for incremental scans
• Add unit coverage for explicit ranges, cursor precedence, timestamp conversion, retention errors, and ambiguous bounds
• Document incremental scanning and update the Stellar scan example

Closes #11

Follow-up

• Filed Spectre scanner follow-up: Use Stellar announcement cursor for incremental background scans spectre#1

Validation

• npx prettier --check .
• npx vitest run
• npx tsc --noEmit
• npx tsup
• git diff --check

Note: local hooks expect pnpm, but pnpm is not available in this shell; the equivalent validation commands above were run with npx.

[truthixify]

Heads up — we've just set up a develop branch as the integration target for all Drips × Stellar Wave contributions, and I've retargeted this PR's base from main to develop on your behalf. The diff is unchanged; nothing to do on your end.

Going forward, please branch from and target develop. We'll periodically merge develop → main as we cut releases. Thanks for contributing!

[truthixify]

Clean API design with function overloads preserving the old call shape. RetentionExceededError typed correctly, all 5 acceptance scenarios covered in tests (explicit range, cursor preference, timestamp conversion, retention bound, ambiguous bounds). Merging. Thanks @kaycke1337.