Add vendored Flatbuffers (v25.9.23) #224

Job	Run time
identifiers / Build GitHub identifiers	10s
identifiers / Log resolved identifiers	4s
Build wheels on ubuntu-24.04 (x86_64)	1m 58s
Build wheels on windows-2022 (x86_64)	3m 40s
Build wheels on macos-15 (arm64)	2m 39s
	8m 31s

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