🧹 Remove unused used field in CodePage

[undivisible]

🎯 What: Removed the used field and its complex #[cfg_attr(..., allow(dead_code))] from CodePage in in-cli/src/jit_runtime.rs. The required byte count is now passed directly as an argument (_used) to the finalize method.
💡 Why: This simplifies the struct definition, removes an unnecessary and clunky conditional compilation attribute for silencing dead code warnings on unsupported platforms (like x86_64), and cleans up initialization logic by making page immutable when loading JIT code.
✅ Verification: Verified by ensuring the code formats with cargo fmt, passes linting via cargo clippy --all-targets --features extended --locked -- -D warnings, and passes both the jit_runtime specific test and the full test suite (cargo test).
✨ Result: A cleaner CodePage structure with better memory layout and safer, immutable initialization without dead-code allowance hacks.

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PR created automatically by Jules for task 16642229359867925454 started by @undivisible

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Note

Low Risk
Localized JIT runtime cleanup with no change to when or how much memory is flushed or made executable.

Overview
Refactors how the JIT runtime tracks how many bytes were written into an executable CodePage.

The used field is removed from CodePage, along with the platform-specific allow(dead_code) attribute that existed because used was only read on macOS and AArch64 Linux for instruction-cache invalidation.

finalize now takes the written byte count (code.len() from JitRuntime::load) instead of reading it from the page. load no longer mutates the page after allocation—only copies code, applies relocations, pushes the page, then calls finalize(code.len()).

Behavior for icache flush and mprotect is unchanged; the count is just supplied at finalize time rather than stored on the struct.

^{Reviewed by Cursor Bugbot for commit 81b3d74. Configure here.}

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