Seeds and target frequency
Make an RTL++ build reproducible with its placement seed, hold every clock to a target frequency, and sweep seeds to see the spread.
Seed
Placement starts from a seed, a whole number from 0 to 2,147,483,647, and 1 unless you set it. The same sources, seed and tools give the same bitstream. The seed is saved in rtlpp.toml, so it travels with the project.
Target frequency
Set Target MHz in the Build lens to hold every clock to that frequency, up to 1,000 MHz. Clocks your constraint file leaves unset take it, and clocks it sets are retargeted for the build; the file in the project does not change. It is saved in rtlpp.toml as freq_mhz. Leave it empty to use the constraint file's own frequencies and nextpnr's default of 12 MHz for the rest.
Seed sweep
Seeds per build in the Inspector runs 1, 4 or 8 builds from consecutive seeds, starting at the project's seed. The timing line then gives each clock's worst, median and best frequency across the seeds, and the view shows the best one.