fix(tracking): weight by-command savings percentage - #3510

Summary

• derive each by-command savings percentage from aggregate saved and input token totals
• label the weighted value as Total% instead of Avg%
• cover weighted long-tail command data and zero-input passthrough records

Closes #3506

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets
• cargo test --all -- --test-threads=1
• targeted regression test confirmed red before the fix (expected 89.2079%, got 50%) and green after it
• manual testing with an isolated RTK_DB_PATH: rtk proxy followed by rtk gain displayed the Total% column

AI assistance was used during investigation or implementation.