🧹 Extract standard library Sys registry closures into functions

[Tcode-Motion]

🎯 What: Extracted the bulky inline closures (like read_file, write_file, exists, now, sleep) inside register_sys into standalone top-level functions (sys_read_file, sys_write_file, etc.). Abstracted the repetitive filesystem capability validation into a check_fs_capability helper function.
💡 Why: Drastically improves the maintainability and readability of stdlib/src/sys.rs. The register_sys function is no longer cluttered with heavy implementation logic, and the security policy enforcement is deduplicated.
✅ Verification: Ran cargo check and cargo test --workspace --all-targets to confirm no regressions were introduced. The standalone functions safely coerce to StdFnCallback function pointers required by StdFunction.
✨ Result: Cleaned up technical debt and improved the legibility of the standard library registry.

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PR created automatically by Jules for task 6510930888015025566 started by @Tcode-Motion

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