Add design docs for behavior guard, audit trail, CI quality gate, and security-aware guard

[benzeneCLI]

This PR adds design/spec documentation under docs/ for four related Soroban optimization-safety features, following this repo's existing convention of substantial markdown design docs (e.g. docs/AUDIT_LOGGING_SYSTEM.md).

Closes #825
Closes #826
Closes #827
Closes #828

• Implement Soroban Optimization Behavior Guard #825: Design for a behavior guard that compares baseline vs. optimized contract execution results (return values, events, storage) and rejects any optimization that changes observable behavior.
• Implement Soroban Optimization Audit Trail #826: Design for an append-only audit trail recording rule id, source diff, confidence score, validation results, and timestamp for every optimization recommendation.
• Implement Soroban Optimization CI Quality Gate #827: Design for a Soroban CI quality gate with configurable thresholds and machine-readable, PR-reportable output, referencing the existing gasguard-scan.yml/ci.yml patterns (doc only, no workflow changes).
• Implement Soroban Security-Aware Optimization Guard #828: Design for a security-aware guard that flags patches touching authorization/state logic and blocks any that fail the existing rules/security/ checks, regardless of resource savings.

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