docs(stellar): add unified STRIDE threat model

[TheWeirdDee]

Summary

• add a unified STRIDE threat model for all four core Stellar contracts
• map every public contract entry point to at least one STRIDE category
• document per-contract attack surfaces and implemented mitigations
• reference relevant findings from existing contract audits
• separate residual risks from mitigated threats
• add contract-level READMEs linking to the unified threat model

Contracts covered

• stealth-announcer
• stealth-registry
• stealth-sender
• wraith-names

Validation

• verified coverage against every public #[contractimpl] entry point
• verified local Markdown links
• ran git diff --check

Documentation-only change; no contract behavior was modified.

Closes #113

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