Allow multiple assets to be updated from same node

[sneakers-the-rat]

Fix: #200

Previously, it was only possible to have one asset dependency mapping per node. However nodes can emit multiple signals, and the same signal might be used to update multiple assets (not exactly sure when this would be useful, but it's a logical implication of the spec and should work).

Simple enough change, using a nested dict to keep the external lookups quick (O(1) is node relevant to the update, as used in the zmq runner) - alternative considered was using NodeSignal() as the key but that requires allocating a tuple on every lookup.

added dependency_assets cached prop to get the set of assets with dependencies since the dependencies dict is no longer 1:1 count of assets in the value field.

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📚 Documentation preview 📚: https://noob--244.org.readthedocs.build/en/244/

[coveralls]

coverage: 84.583% (-0.006%) from 84.589% — issue-200 into main

[codspeed-hq]

Merging this PR will degrade performance by 13.55%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 (👁 1) regressed benchmark
✅ 7 untouched benchmarks
⏩ 8 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
👁	Simulation	test_kitchen_sink_process[ZMQRunner-testing-kitchen-sink]	2.8 ms	3.2 ms	-13.55%

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_{Comparing issue-200 (ad058b1) with main (5c236d7)}

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. ↩

[sneakers-the-rat]

perf regression is fake news, genuinely hate that thing