add codecov badge #254

Job	Run time
coverage - measure	1m 47s
sanitizer - arm32v7/debian:bookworm	3m 8s
linux - debian:bookworm	2m 52s
sanitizer - arm64v8/debian:bookworm	2m 35s
ios	1m 2s
macosx	2m 25s
sanitizer - debian:bookworm	1m 40s
linux - arm64v8/debian:bookworm	2m 41s
linux - arm32v7/debian:bookworm	2m 48s
android	4m 53s
linux - ubuntu:noble	1m 36s
vcpkg - windows	2m 44s
conan - macos-latest	2m 53s
vcpkg - linux \|	1m 44s
conan - linux \|	2m 2s
msys2 - mingw32	2m 24s
conan - emscripten \|	2m 2s
msys2 - mingw64	2m 15s
msvs	4m 22s
	47m 53s

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