[WIP] initial fixpoint iteration

[carljm]

Pulls in a Salsa version that supports fixpoint iteration, for experimentation.

[codspeed-hq]

CodSpeed Performance Report

Merging #14029 will degrade performances by 11.21%

_{Comparing cjm/fixpoint (3189bd8) with main (b89d61b)}

Summary

❌ 1 regressions
✅ 31 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	red_knot_check_file[incremental]	5.5 ms	6.2 ms	-11.21%

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[MichaReiser]

Nice on bringing down the regression. I'm still somewhat surprised that we see an 11% perf regression and a 4 perf regression in the salsa benchmarks (that don't have any cycles).

[carljm]

Yeah I planned to do some more looking at profiles; thanks for doing that!