fix: keep product overview diagrams on the native path

[baokhang83]

Summary

• make the product overview's prose and diagram paths explicit
• require the native renderer instead of hand-written HTML, SVG, or CSS diagrams
• add a bounded merge layout for independent inputs and short chains converging on one result
• render relationship labels and match the embedded site frame height, preventing blank lower panels

Verification

• ShellCheck and Node syntax checks
• bats tests/diagram-renderer.bats tests/plugin.bats tests/cli.bats