fix(encode): adjust max pack size for 32-bit targets to prevent overflow

[yueneiqi]

When run buck2 build //... --target-platforms //platforms:i686-unknown-linux-gnu on ci, encounter:

error: this arithmetic operation will overflow
   --> vendor/src/internal/pack/encode.rs:758:18
    |
758 |             Some(1024 * 1024 * 1024 * 6), // 6GB
    |                  ^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `1073741824_usize * 6_usize`, which would overflow
    |
    = note: `#[deny(arithmetic_overflow)]` on by default

This PR fixes that.

Summary

• make pack header size check 32-bit safe
• adjust max pack size for 32-bit targets to prevent overflow

Testing

• cargo test
• cargo test --target i686-unknown-linux-gnu
• ci log

---

This PR was primarily authored with Codex using GPT-5.2-Codex and then hand-reviewed by me. I AM responsible for every change made in this PR. I aimed to keep it aligned with our goals, though I may have missed minor issues. Please flag anything that feels off, I'll fix it quickly.

[genedna]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

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[Copilot]

Pull request overview

This PR addresses 32-bit platform safety by preventing integer overflow in pack size calculations. The changes ensure that pack operations don't cause panics on 32-bit targets where usize is limited to ~4GB.

Key changes:

• Modified cache memory size calculation to use wider arithmetic (u128) preventing overflow when computing 80% of memory limit
• Adjusted test pack size limits to use 2GB on 32-bit targets instead of 6GB to fit within usize bounds
• Added a regression test for 32-bit overflow scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/internal/pack/decode.rs	Fixed cache memory calculation to prevent overflow using u128 intermediate type; added 32-bit specific test case
src/internal/pack/encode.rs	Added platform-specific pack size limits in test helper to avoid overflow on 32-bit targets