FE: Builds comparison table

[alanpeixinho]

Summary

Display the row-level comparison table for builds inside the comparison view.

Details

Table content

• Shows all build rows where status_a ≠ status_b, including rows that ran on only one side (the other side is null)
• Columns: Config, Architecture, Compiler, Side A status, Side B status

Filtering

• Filter by transition type: regressions (PASS→FAIL), fixes (FAIL→PASS), all changes, appeared/disappeared (null on one side)

Grouping

• Groupable by config or architecture
• Grouped view shows flip counts per group

Empty state

• When no builds differ, show the "no changes" state consistent with the comparison page

Notes

• Same UX pattern as the boots/tests table (FE: Boots/Tests comparison table #2011), but different columns reflecting the build identity key (config + arch + compiler)
• Simpler than boots/tests (fewer rows typically, no path hierarchy)
• null side rendered as a neutral/empty state, consistent with FE: Boots/Tests comparison table #2011

[mentonin]

Consider making compiler groupable as well