Fix BigMath.atan precision safe margin #433

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-latest 2.7	1m 6s
ubuntu-latest 3.0	34s
ubuntu-latest 3.1	36s
ubuntu-latest 3.2	40s
ubuntu-latest 3.3	36s
ubuntu-latest 3.4	32s
ubuntu-latest debug	42s
ubuntu-latest head	31s
macos-latest 2.7	53s
macos-latest 3.0	36s
macos-latest 3.1	30s
macos-latest 3.2	22s
macos-latest 3.3	21s
macos-latest 3.4	21s
macos-latest debug	29s
macos-latest head	39s
macos-14 2.7	55s
macos-14 3.0	43s
macos-14 3.1	29s
macos-14 3.2	38s
macos-14 3.3	24s
macos-14 3.4	21s
macos-14 debug	50s
macos-14 head	25s
windows-latest 2.7	5m 13s
windows-latest 3.3	3m 9s
windows-latest 3.4	3m 45s
windows-latest head	3m 23s
windows-latest mingw	3m 5s
windows-latest mswin	2m 34s
	35m 24s

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