[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 65.26%

_{Comparing cjm/fixpoint (8da1ec2) with main (b8acadd)}

Summary

❌ 1 regressions
✅ 31 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	cjm/fixpoint	Change
❌	red_knot_check_file[incremental]	4.6 ms	13.3 ms	-65.26%

[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.