hdl._dsl: use Enum for fsm state signal #3596

Job	Run time
check-links	46s
smoke (amaranth-lang/amaranth-boards)	33s
test (3.9)	2m 15s
smoke (amaranth-lang/amaranth-stdio)	56s
test (3.10)	2m 4s
smoke (amaranth-lang/amaranth-soc)	58s
test (3.11)	2m 42s
test (3.12)	3m 31s
document	50s
test (3.13)	2m 59s
test (pypy-3.9)	6m 16s
test (pypy-3.10)	4m 38s
test (3.14-dev)	1m 38s
publish-docs	6s
publish-docs-dev	0s
required	1s
publish-schemas	6s
publish-package	40s
publish-release	0s
	30m 59s

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