Soroban contract test coverage measurement + CI gate

[truthixify]

Tier: M (2-4 days) Type: test

Context

No coverage measurement today. For security-critical Stellar contracts we should know what % of branches the test suite touches and prevent regressions.

Scope

• Set up cargo-tarpaulin (or cargo-llvm-cov) for the Soroban workspace
• Target 90%+ coverage for stealth-announcer, stealth-registry, stealth-sender
• Target 80%+ for wraith-names and stealth-splitter
• CI gate fails when coverage drops below threshold
• HTML report uploaded as artifact

Acceptance criteria

• Coverage tooling integrated
• Per-contract threshold enforcement in CI
• First-run report committed as baseline
• Documentation in stellar/README.md

Files to start with

• stellar/Cargo.toml (workspace config)
• .github/workflows/