convert signals from a list to a dict to match slots

[sneakers-the-rat]

For some reason the return type of slots was dict[str, Slot] and the return type of signals was list[Signal].

pointlessly different signature for two closely related things. just make them the same.

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📚 Documentation preview 📚: https://noob--212.org.readthedocs.build/en/212/

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing signals-as-dict (05e6bb4) with main (f046a27)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩