feat: detect missing NatSpec and external calls in loops

[Leothosine]

Summary

• Added MissingNatspecRule that flags public functions without NatSpec doc comments and suggests the @notice/@param/@return template.
• Added ExternalCallsInLoopsRule that detects external calls (transfer/call/send/invoke) inside for/while loops and warns about gas and reentrancy risk.

Changes

• packages/rules/src/auditability/docs/missing_natspec.rs
• packages/rules/src/auditability/docs/mod.rs
• packages/rules/src/auditability/mod.rs
• packages/rules/src/security/external_calls/external_calls_in_loops.rs
• packages/rules/src/security/external_calls/mod.rs

closes #316
closes #318

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