hdl._dsl: use Enum for fsm state signal #3597

Job	Run time
smoke (amaranth-lang/amaranth-boards)	39s
check-links	50s
smoke (amaranth-lang/amaranth-stdio)	1m 0s
document	50s
smoke (amaranth-lang/amaranth-soc)	1m 1s
test (3.9)	2m 17s
test (3.10)	2m 11s
test (3.11)	2m 23s
test (3.12)	2m 55s
test (3.13)	2m 41s
test (pypy-3.9)	6m 4s
test (pypy-3.10)	4m 22s
test (3.14-dev)	1m 31s
publish-docs	13s
publish-docs-dev	0s
required	1s
publish-package	51s
publish-schemas	17s
publish-release	0s
	30m 6s

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