perf(tui): reduce hot path allocations

[kkyosuke]

Summary

• Make issue CLI progress calculations overflow-safe by scaling in u128 before division.
• Make TUI progress bar scaling overflow-safe for pathological counts or widths.
• Add regression tests for counts near usize::MAX.

Why

The review found a theoretical overflow risk in progress calculations that multiplied usize values before dividing. In debug builds this can panic for huge counts; in release builds it can wrap. Normal project sizes are unaffected, but using a wider intermediate type removes the hazard without changing visible output.

Validation

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test --quiet — 1835 passed

[github-actions]

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