ENH: implement `FrozenDict` with `frozendict` #42

Job	Run time
lock / Find lock files	4s
lock / Check if PR already exists	0s
lock / Manifest.toml	0s
lock / Determine Python versions	0s
lock / pixi.lock	0s
lock / .pre-commit-config.yaml	19s
lock / uv.lock	12s
lock / pip constraint files	0s
lock / Push changes	7s
lock / Create PR	0s
	42s

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