Add gas-optimized reentrancy guard, G006/G003 rules, gas snapshot diff script

[Leothosine]

Adds small, focused starter implementations for four assigned issues:

• GasOptimizedReentrancyGuard: 1/2 sentinel reentrancy guard so re-entry checks hit a warm slot instead of a cold 0->1 write.
• detect_missing_payable (Rule G006): flags onlyOwner/onlyAdmin functions missing payable.
• detect_public_vars_in_abstract (Rule G003): flags public state variable declarations intended for abstract/internal contracts.
• compare-gas-snapshots.sh: diffs .gas-snapshot against a baseline ref and prints a markdown diff table. Wiring this into .github/workflows/gas-diff.yml is a follow-up, since pushing workflow file changes requires a token with the workflow scope.

These are intentionally small starter implementations covering the core of each acceptance criteria; dedicated test fixtures and full AST/CFG-based analysis can follow in subsequent PRs.

Closes #652
Closes #651
Closes #650
Closes #649

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