AP_InertialNav: add fallback numeric vert velocity #32734

Job	Run time
build (stm32f7, chibios, 10)	21m 25s
build (stm32h7, chibios, 10)	17m 47s
build (fmuv2-plane, chibios, 10)	9m 47s
build (periph-build, chibios, 10)	40m 8s
build (iofirmware, chibios, 10)	3m 11s
build (CubeOrange-bootloader, chibios, 10)	1m 47s
build (fmuv3-bootloader, chibios, 10)	1m 29s
build (revo-bootloader, chibios, 10)	1m 59s
build (stm32h7-debug, chibios, 10)	10m 17s
build (fmuv3, chibios, 10)	22m 2s
build (revo-mini, chibios, 10)	17m 26s
build (MatekF405-Wing, chibios, 10)	19m 17s
build (CubeOrange-ODID, chibios, 10)	13m 26s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 47s
build (configure-all, chibios, 10)	12m 23s
build (build-options-defaults-test, chibios, 10)	30m 39s
build (signing, chibios, 10)	13m 39s
	3h 58m 29s

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