test(coverage): enforce per-file coverage thresholds for critical Stellar modules (#1003)

[yodakkak]

Closes #1003

Summary

• Configured criticalPatterns in �itest.config.js with >=90% lines and branches thresholds for src/lib/stellar*, src/lib/transactionBuilder.ts, src/lib/wallet/**, and src/lib/multisig*.
• Updated scripts/check-coverage.mjs with matchesCriticalPattern, evaluateCoverageSummary, and ormatCoverageMarkdownSummary to report per-file failures to $GITHUB_STEP_SUMMARY.
• Added unit tests in ests/unit/scripts/check-coverage.test.js covering primary, boundary (90%), and failure (<90%) cases.

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