Mainnet launch readiness checklist + go/no-go tracking

[Nemenwq]

I have completed the creation of the Stellar mainnet readiness checklist. You can find it at contracts/stellar/MAINNET_READINESS.md. I've also provided a walkthrough of the document.

Closes #21

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@Nemenwq Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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[truthixify]

The checklist hits the right shape — code quality / ops / ecosystem / communication / risk / sign-off sections all there, the "Known Limitations of v1" subsection is a really good thing to track explicitly.

Two follow-ups worth noting (not blocking the merge):

1. The issue references are written as [#01](#01) etc., which are anchor links to nonexistent headings on this page. GitHub will auto-link bare #1, #5, #17 etc. to the actual issues if you just write them inline — Audit #1, Audit #5, etc. Cleaner and they actually work.
2. There's also a MAINNET_READINESS.md at repo root that landed in Stuck-funds rescue mechanism for stealth-sender FIXED #28. Yours at stellar/MAINNET_READINESS.md is the right path per the spec; we'll consolidate the two in a follow-up.

Thanks @Nemenwq.