Require that happens_after is not mutable #3229

Job	Run time
Mypy	1m 24s
Pylint	1m 36s
Tests for downstream project Firedrake	16m 20s
Documentation	1m 30s
Tests for downstream project arraycontext	2m 21s
Conda Examples	1m 48s
Tests for downstream project meshmode	9m 42s
Conda Pytest (ubuntu-latest)	3m 22s
Conda Pytest with Intel CL	3m 52s
Tests for downstream project grudge	10m 3s
Conda Pytest (macos-latest)	3m 2s
Tests for downstream project pytential	51m 29s
Conda Pytest without arg check	3m 48s
Tests for downstream project pytato	5m 10s
Conda Pytest Twice (for cache behavior)	3m 41s
Ruff	6s
Typos	4s
Validate CITATION.cff	6s
	1h 59m 24s

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