fix(core): preserve table width remainders

[Bortlesboat]

Summary:

• Preserve leftover width when proportional table allocation floors fractional shares.
• Add a regression test for the in-between width case where the old logic rendered narrower than available space.

Verification:

• cargo test -p cortex-core test_column_width_distribution_preserves_available_space -- --nocapture
• cargo test -p cortex-core markdown::table -- --nocapture
• cargo test -p cortex-core --lib
• cargo check -p cortex-core
• cargo fmt --check --package cortex-core
• git diff --check

Related: PlatformNetwork/bounty-challenge#53337

[greptile-apps]

Your free trial has ended. If you'd like to continue receiving code reviews, you can add a payment method here.