Validate entrypoint account bounds

[edo-chan]

Summary

• reject top-level instructions with more than MAX_ACCOUNTS accounts before filling fixed account/classification arrays
• validate each fixed-array write slot in the entrypoint account loop
• reject duplicate account references that point to the current or a future account slot before reading from initialized account storage
• add focused unit coverage for account-count, slot, and duplicate-index validation

Security note

This addresses the program-entry side of oversized account-list handling. It is separate from #165, which fixes compact-instruction InstructionHolder account metadata ownership.

Tests

• rustup run nightly rustfmt --edition 2021 program/src/lib.rs
• cargo test -p swig validate_ --lib
• cargo test -p swig duplicate_account --lib
• cargo +stable clippy -p swig --lib --tests

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