perf: share compiled read paths across r/z-variables and CDF v2 files

[Beforerr]

Lower time to first read for r-variable files (ACE, OMNI2) and CDF v2 files. Measured in fresh sessions on Julia 1.13.

first use	before	after
CDF v2 ds[name] (ac_h2_sis)	487 ms	3.6 ms
CDF v2 first attribute access (FILLVAL)	46 ms	0.04 ms
CDF v2 Array (Float32 2-D)	63 ms	4 ms
r-var Float32 1-D: attrib + Array + v[1] + v[:,1:1]	~45 ms	~0.4 ms
r-var Epoch: attrib + Array + v[1] + v[:,1:1]	~100 ms	~17 ms

• rVDR is merged into VDR, with a new zvar flag. CDFVariable no longer specializes on the variable kind, so r-variables reuse the z-variable specializations.
• LABL_PTR_* resolution uses Array, which skips DiskArrays indexing. That indexing compiles again for every string width (~60 ms each).
• The precompile workload now also covers a v2 file (ac_h2_sis), and it compiles Dict(var.attrib) instead of a single get.

Cost: precompile takes about 1 s longer (~3.5 s → 4.5 s), and the package image grows from 8.7 MB to 10.1 MB. Load time is unchanged at ~66 ms.

Remaining cost, not addressed here: every new StaticString{N} width or unseeded element type (e.g. Int32 1-D) still costs ~30–100 ms for DiskArrays indexing (v[1], v[:,1:1]). Array(var) avoids it.

[codecov]

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[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	b5f40f2...	main / b5f40f2...
elx/full_load	23.4 ± 1.2 μs	17.4 ± 1.2 μs	1.34 ± 0.11
elx/sum_array	0.852 ± 0.47 μs	1.09 ± 0.61 μs	0.78 ± 0.61
elx/sum_lazy	1.15 ± 0.24 μs	1.13 ± 0.31 μs	1.02 ± 0.35
elx/sum_var_access	0.912 ± 0.49 μs	0.961 ± 0.6 μs	0.949 ± 0.78
elx/var_access	0.13 ± 0.01 μs	0.13 ± 0.01 μs	1 ± 0.11
epochs/DateTime_to_TT2000	29.2 ± 1.6 μs	29.6 ± 1.5 μs	0.986 ± 0.071
epochs/Epoch16_parse	0.166 ± 0.0058 ms	0.168 ± 0.0042 ms	0.991 ± 0.042
epochs/Epoch16_vs_DateTime	0.772 ± 0.011 μs	0.781 ± 0.01 μs	0.988 ± 0.019
epochs/TT2000_parse	0.189 ± 0.0068 ms	0.192 ± 0.0075 ms	0.987 ± 0.052
epochs/TT2000_string	3.44 ± 1.7 ms	3.36 ± 1.6 ms	1.02 ± 0.69
epochs/TT2000_to_DateTime	0.0566 ± 0.00065 ms	0.0572 ± 0.00053 ms	0.989 ± 0.015
epochs/TT2000_vs_DateTime	25.5 ± 0.12 μs	27.9 ± 0.5 μs	0.915 ± 0.017
mms/full_load	15.2 ± 1.5 μs	13.1 ± 1.4 μs	1.16 ± 0.16
mms/sum_array	0.0617 ± 0.0018 s	0.0608 ± 0.0015 s	1.02 ± 0.039
mms/sum_slice	2.23 ± 0.12 ms	2.22 ± 0.31 ms	1.01 ± 0.15
mms/sum_var_access	2.23 ± 0.13 ms	2.2 ± 0.32 ms	1.02 ± 0.16
mms/var_access	0.06 ± 0.01 μs	0.05 ± 0.01 μs	1.2 ± 0.31
time_to_load	0.116 ± 0.0017 s	0.117 ± 0.01 s	0.989 ± 0.086

Memory benchmarks

	main	b5f40f2...	main / b5f40f2...
elx/full_load	0.217 k allocs: 24.8 kB	0.217 k allocs: 24.8 kB	1
elx/sum_array	12 allocs: 28 kB	12 allocs: 28 kB	1
elx/sum_lazy	12 allocs: 28 kB	12 allocs: 28 kB	1
elx/sum_var_access	14 allocs: 28.3 kB	14 allocs: 28.3 kB	1
elx/var_access	1 allocs: 0.234 kB	1 allocs: 0.234 kB	1
epochs/DateTime_to_TT2000	3 allocs: 0.0764 MB	3 allocs: 0.0764 MB	1
epochs/Epoch16_parse	3 allocs: 0.153 MB	3 allocs: 0.153 MB	1
epochs/Epoch16_vs_DateTime	0 allocs: 0 B	0 allocs: 0 B
epochs/TT2000_parse	3 allocs: 0.0764 MB	3 allocs: 0.0764 MB	1
epochs/TT2000_string	0.2 M allocs: 7.02 MB	0.2 M allocs: 7.02 MB	1
epochs/TT2000_to_DateTime	3 allocs: 0.0764 MB	3 allocs: 0.0764 MB	1
epochs/TT2000_vs_DateTime	0 allocs: 0 B	0 allocs: 0 B
mms/full_load	0.065 k allocs: 6.45 kB	0.065 k allocs: 6.45 kB	1
mms/sum_array	1.04 k allocs: 31.7 MB	1.04 k allocs: 31.7 MB	1
mms/sum_slice	0.069 k allocs: 1.29 MB	0.069 k allocs: 1.29 MB	1
mms/sum_var_access	0.069 k allocs: 1.29 MB	0.069 k allocs: 1.29 MB	1
mms/var_access	1 allocs: 0.219 kB	1 allocs: 0.219 kB	1
time_to_load	0.2 k allocs: 11.8 kB	0.2 k allocs: 11.8 kB	1