Rust hot path

What to change in the code after a profile names the hot spot. The rust-performance skill produces the profile and owns benchmark setup; this skill turns the profile into a diff. Apply a rule here only when a measurement points at it: optimized code is harder to read, so each change must pay for the readability it spends.

Figures were measured on rustc 1.97.0, aarch64 and x86_64, unless a line names another version. Std growth policy and layout are unspecified implementation details. Re-measure them on your toolchain before you depend on an exact figure.

Route the profile to a section