Add benchmarks and optimize diagnose overhead

[kalverra]

Diagnose Overhead Benchmarks

just bench_overhead_matrix

This runs a few benchmark tests to measure the overhead the diagnose command adds to go test.

--------------------------------------------------------------------------------
Diagnose overhead vs raw go test (dogfood, target=./..., 5-run average per cell)
--------------------------------------------------------------------------------
iters  parallel  baseline    diagnose    overhead   overhead%  overhead/iter
1      1         1.618418s   1.566522s   0s         0.0%       0s
4      1         6.165101s   6.357416s   192.316ms  3.0%       48.079ms
4      4         4.994151s   5.099707s   105.555ms  2.1%       26.389ms
8      1         12.500664s  12.682847s  182.184ms  1.4%       22.773ms
8      8         9.77423s    9.753534s   0s         0.0%       0s

When I first ran the benchmark, overhead was a bit large (~50MB memory + ~7% of execution time), and so I had claude run some experiments and validate improvements with the benchmark. Memory overhead was cut to ~5MB and execution time to ~2.5%.

Also removes a bunch of dead code

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Pull request overview

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