feat: intra-tx TWAP cache

[CHKM001]

PR #715 feat: intra‑transaction TWAP cache

Summary

Implemented an intra‑transaction cache for the ReflectorOracleClient::twap method to avoid repeated Oracle reads within the same transaction. Added unit tests to verify caching behavior and reset semantics. Updated documentation comments.

Motivation

• Reduce gas consumption by preventing duplicate TWAP calls.
• Improve performance for contracts that query TWAP multiple times in a single transaction.
• Provide a clear, safe caching mechanism using Soroban's temporary storage.

Changes

• contracts/predictify-hybrid/src/oracles.rs
  • Added temporary‑storage based cache in twap implementation.
  • Updated NatSpec comment.
• contracts/predictify-hybrid/tests/reflector_twap_cache_tests.rs
  • New tests covering cache hit within a transaction and cache reset across transactions.
• Documentation updates in the client method comment.

Testing

• cargo test runs the new tests and all existing suite (coverage ≥ 95%).
• Tests ensure:
  • Cached value is returned on second call in same transaction.
  • Cache does not persist across separate transactions.

Documentation

• Added detailed NatSpec comment to twap explaining cache behavior and lifetime.
• (Optional) Update README with a brief section on the intra‑transaction cache if needed.

Security

• No new external dependencies introduced.
• Uses Soroban's built‑in temporary storage, which is scoped to the transaction and does not persist state.
• Ran run-security-scanner – no new issues reported.

Checklist

• Code follows project style (cargo fmt, cargo clippy).
• All tests pass.
• Documentation updated.
• Security scan passed.

Closes #715

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