Use link capacity for graph scaling

[domcyrus]

Summary

• Read native interface link capacity and use it as a stable RX/TX graph reference.
• Keep low utilization visible with a fixed curve, with adaptive scaling as the fallback.

Linked issue

None.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo build --release
• cargo audit

Scope

• Focused change with no new dependencies
• Tests, documentation, and the Unreleased changelog are updated