hdl._dsl: use `Enum` for fsm state signal #3588

Job	Run time
check-links	49s
smoke (amaranth-lang/amaranth-boards)	42s
document	1m 0s
test (3.9)	2m 23s
smoke (amaranth-lang/amaranth-stdio)	54s
test (3.10)	2m 32s
smoke (amaranth-lang/amaranth-soc)	1m 3s
test (3.11)	2m 47s
test (3.12)	3m 27s
test (3.13)	3m 18s
test (pypy-3.9)	6m 30s
test (pypy-3.10)	4m 49s
test (3.14-dev)	1m 49s
publish-docs	5s
publish-docs-dev	0s
required	1s
publish-package	39s
publish-schemas	10s
publish-release	0s
	32m 58s

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