go-tools f55da7c: mutate shares the machine and reports its costs

[aledbf]

Takes go-tools f55da7c (#12-#14), which only changes the mutation gate:

• each test process it starts is held to its share of the cores (GOMAXPROCS = cores / -j): a control-plane change at -j 4 went from a load of 24 to 11 on a 20-core machine, and -j 10 from 4m13s to 1m56s;
• the run ends with where its time went (processes, wall, CPU per purpose) and the ten tests the edits were asked of the longest in all.

go.mod and go.sum only; lint (with the new tools) green.

🤖 Generated with Claude Code

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