CI: upload Codecov only after all test jobs finish

[MilesCranmerBot]

This changes CI so Codecov upload happens in a dedicated final job that waits for the full test matrix to complete.

• Each test job now uploads its generated lcov.info as a GitHub Actions artifact.
• A new codecov job (needs: [test, additional_tests]) downloads all artifacts, concatenates the lcov.info files into merged-lcov.info, and uploads once to Codecov.

This should prevent spurious Codecov failures caused by uploads happening while the matrix is still running.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	7dccac6...	master / 7dccac6...
eval/ComplexF32/evaluation	6.91 ± 0.49 ms	6.96 ± 0.48 ms	0.993 ± 0.097
eval/ComplexF64/evaluation	10.3 ± 0.78 ms	10.3 ± 0.85 ms	0.998 ± 0.11
eval/Float32/derivative	11.3 ± 0.94 ms	11.5 ± 0.86 ms	0.982 ± 0.11
eval/Float32/derivative_turbo	11.2 ± 0.9 ms	11.6 ± 0.98 ms	0.963 ± 0.11
eval/Float32/evaluation	2.45 ± 0.25 ms	2.43 ± 0.24 ms	1.01 ± 0.14
eval/Float32/evaluation_bumper	0.598 ± 0.016 ms	0.604 ± 0.016 ms	0.991 ± 0.038
eval/Float32/evaluation_turbo	0.51 ± 0.028 ms	0.514 ± 0.03 ms	0.991 ± 0.08
eval/Float32/evaluation_turbo_bumper	0.599 ± 0.017 ms	0.606 ± 0.016 ms	0.989 ± 0.038
eval/Float64/derivative	14.2 ± 1 ms	14.6 ± 1 ms	0.971 ± 0.096
eval/Float64/derivative_turbo	14.3 ± 1 ms	14.8 ± 1.2 ms	0.97 ± 0.11
eval/Float64/evaluation	2.87 ± 0.31 ms	2.92 ± 0.3 ms	0.984 ± 0.15
eval/Float64/evaluation_bumper	1.27 ± 0.045 ms	1.27 ± 0.046 ms	1 ± 0.051
eval/Float64/evaluation_turbo	1.01 ± 0.065 ms	1.02 ± 0.06 ms	0.99 ± 0.087
eval/Float64/evaluation_turbo_bumper	1.26 ± 0.044 ms	1.26 ± 0.045 ms	0.997 ± 0.05
utils/combine_operators/break_sharing	0.0403 ± 0.0008 ms	0.0406 ± 0.00093 ms	0.994 ± 0.03
utils/convert/break_sharing	27.5 ± 3 μs	27.1 ± 3.3 μs	1.01 ± 0.17
utils/convert/preserve_sharing	0.0987 ± 0.0077 ms	0.0989 ± 0.0081 ms	0.998 ± 0.11
utils/copy/break_sharing	27.4 ± 3.1 μs	27.2 ± 3.1 μs	1 ± 0.16
utils/copy/preserve_sharing	0.0994 ± 0.0085 ms	0.0983 ± 0.008 ms	1.01 ± 0.12
utils/count_constant_nodes/break_sharing	12.6 ± 0.73 μs	12.5 ± 0.93 μs	1 ± 0.095
utils/count_constant_nodes/preserve_sharing	0.084 ± 0.0057 ms	0.0837 ± 0.0059 ms	1 ± 0.098
utils/count_depth/break_sharing	13.4 ± 0.93 μs	13.6 ± 1.2 μs	0.982 ± 0.11
utils/count_nodes/break_sharing	12.3 ± 0.81 μs	12.2 ± 0.75 μs	1.01 ± 0.091
utils/count_nodes/preserve_sharing	0.0853 ± 0.0057 ms	0.085 ± 0.0059 ms	1 ± 0.097
utils/get_set_constants!/break_sharing	0.0321 ± 0.0034 ms	0.0323 ± 0.0031 ms	0.992 ± 0.14
utils/get_set_constants!/preserve_sharing	0.172 ± 0.0094 ms	0.172 ± 0.0099 ms	1 ± 0.079
utils/get_set_constants_parametric	0.0455 ± 0.0057 ms	0.0465 ± 0.005 ms	0.978 ± 0.16
utils/has_constants/break_sharing	7.04 ± 0.58 μs	7.08 ± 0.66 μs	0.994 ± 0.12
utils/has_operators/break_sharing	2.58 ± 0.12 μs	2.5 ± 0.12 μs	1.03 ± 0.069
utils/hash/break_sharing	23.6 ± 1.3 μs	23.8 ± 2.2 μs	0.994 ± 0.11
utils/hash/preserve_sharing	0.0994 ± 0.0065 ms	0.098 ± 0.0061 ms	1.01 ± 0.091
utils/index_constant_nodes/break_sharing	28.5 ± 1.8 μs	28.1 ± 1.9 μs	1.01 ± 0.094
utils/index_constant_nodes/preserve_sharing	0.1 ± 0.007 ms	0.0999 ± 0.0073 ms	1.01 ± 0.1
utils/is_constant/break_sharing	7.7 ± 0.61 μs	7.55 ± 0.62 μs	1.02 ± 0.12
utils/simplify_tree/break_sharing	30 ± 1.2 μs	30.1 ± 1.2 μs	0.996 ± 0.057
utils/simplify_tree/preserve_sharing	0.111 ± 0.0069 ms	0.111 ± 0.0055 ms	0.999 ± 0.079
utils/string_tree/break_sharing	0.434 ± 0.016 ms	0.435 ± 0.017 ms	0.998 ± 0.054
utils/string_tree/preserve_sharing	0.524 ± 0.017 ms	0.532 ± 0.019 ms	0.984 ± 0.046
time_to_load	0.169 ± 0.0013 s	0.17 ± 0.00078 s	0.991 ± 0.009

Memory benchmarks

	master	7dccac6...	master / 7dccac6...
eval/ComplexF32/evaluation	0.99 k allocs: 2.53 MB	0.96 k allocs: 2.46 MB	1.03
eval/ComplexF64/evaluation	0.996 k allocs: 5.07 MB	0.993 k allocs: 5.06 MB	1
eval/Float32/derivative	4.65 k allocs: 17.5 MB	4.65 k allocs: 17.5 MB	1
eval/Float32/derivative_turbo	4.59 k allocs: 17.3 MB	4.67 k allocs: 17.6 MB	0.982
eval/Float32/evaluation	0.981 k allocs: 1.28 MB	0.984 k allocs: 1.28 MB	0.997
eval/Float32/evaluation_bumper	0.303 k allocs: 0.393 MB	0.303 k allocs: 0.393 MB	1
eval/Float32/evaluation_turbo	0.951 k allocs: 1.24 MB	0.957 k allocs: 1.25 MB	0.994
eval/Float32/evaluation_turbo_bumper	0.303 k allocs: 0.393 MB	0.303 k allocs: 0.393 MB	1
eval/Float64/derivative	4.81 k allocs: 0.0352 GB	4.77 k allocs: 0.0349 GB	1.01
eval/Float64/derivative_turbo	4.76 k allocs: 0.0348 GB	4.81 k allocs: 0.0352 GB	0.99
eval/Float64/evaluation	0.996 k allocs: 2.55 MB	1 k allocs: 2.57 MB	0.994
eval/Float64/evaluation_bumper	0.303 k allocs: 0.771 MB	0.303 k allocs: 0.771 MB	1
eval/Float64/evaluation_turbo	0.993 k allocs: 2.54 MB	1 k allocs: 2.57 MB	0.991
eval/Float64/evaluation_turbo_bumper	0.303 k allocs: 0.771 MB	0.303 k allocs: 0.771 MB	1
utils/combine_operators/break_sharing	4 allocs: 0.953 kB	4 allocs: 0.953 kB	1
utils/convert/break_sharing	2 k allocs: 0.123 MB	2 k allocs: 0.123 MB	1
utils/convert/preserve_sharing	2.4 k allocs: 0.192 MB	2.4 k allocs: 0.192 MB	1
utils/copy/break_sharing	2 k allocs: 0.123 MB	2 k allocs: 0.123 MB	1
utils/copy/preserve_sharing	2.4 k allocs: 0.192 MB	2.4 k allocs: 0.192 MB	1
utils/count_constant_nodes/break_sharing	4 allocs: 0.953 kB	4 allocs: 0.953 kB	1
utils/count_constant_nodes/preserve_sharing	0.404 k allocs: 0.0696 MB	0.404 k allocs: 0.0696 MB	1
utils/count_depth/break_sharing	4 allocs: 0.953 kB	4 allocs: 0.953 kB	1
utils/count_nodes/break_sharing	4 allocs: 0.953 kB	4 allocs: 0.953 kB	1
utils/count_nodes/preserve_sharing	0.404 k allocs: 0.0696 MB	0.404 k allocs: 0.0696 MB	1
utils/get_set_constants!/break_sharing	0.898 k allocs: 25.2 kB	0.898 k allocs: 25.2 kB	1
utils/get_set_constants!/preserve_sharing	1.7 k allocs: 0.138 MB	1.7 k allocs: 0.138 MB	1
utils/get_set_constants_parametric	1.42 k allocs: 0.0663 MB	1.42 k allocs: 0.0663 MB	1
utils/has_constants/break_sharing	4 allocs: 0.203 kB	4 allocs: 0.203 kB	1
utils/has_operators/break_sharing	4 allocs: 0.203 kB	4 allocs: 0.203 kB	1
utils/hash/break_sharing	0.104 k allocs: 2.52 kB	0.104 k allocs: 2.52 kB	1
utils/hash/preserve_sharing	0.504 k allocs: 0.0711 MB	0.504 k allocs: 0.0711 MB	1
utils/index_constant_nodes/break_sharing	2.1 k allocs: 0.094 MB	2.1 k allocs: 0.094 MB	1
utils/index_constant_nodes/preserve_sharing	2.5 k allocs: 0.163 MB	2.5 k allocs: 0.163 MB	1
utils/is_constant/break_sharing	4 allocs: 0.203 kB	4 allocs: 0.203 kB	1
utils/simplify_tree/break_sharing	0.104 k allocs: 2.52 kB	0.104 k allocs: 2.52 kB	1
utils/simplify_tree/preserve_sharing	0.504 k allocs: 0.0711 MB	0.504 k allocs: 0.0711 MB	1
utils/string_tree/break_sharing	11.9 k allocs: 0.999 MB	11.9 k allocs: 0.999 MB	1
utils/string_tree/preserve_sharing	12.3 k allocs: 1.07 MB	12.3 k allocs: 1.07 MB	1
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1