feat(soroban): call-frequency analyzer, CPU cost estimator, opt previ…

[Cedarich]

Closes #802, #808, #807, #806

Summary

Implements four Soroban optimization capabilities for GasGuard:

1. Call frequency analyzer — tracks internal helper call relationships, counts repeats, surfaces hot paths
2. CPU cost estimator — ranks expensive computational patterns with relative cost scores
3. Optimization preview API — returns proposed fixes (confidence, diffs, estimated impact) without modifying source
4. Regression checker — re-runs analysis after fixes and flags newly introduced high/critical findings

Acceptance

• Call frequency analyzer + relationships + repeated paths + findings
• CPU cost estimator + expensive patterns + estimates in findings + tests
• Preview API + confidence + diffs + impact + severity filter + API tests
• Regression checker + post-fix analysis + new-finding detection + report

Test

npm test -- call-frequency
npm test -- cpu-cost
npm test -- optimization-preview
npm test -- optimization-regression

closes #802
closes #807
closes #806
closes #808

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