feat!: read CHAR variables as String

[Beforerr]

Replaces StaticString{N,UInt8} with String as the eltype of CDF_CHAR/CDF_UCHAR variables. StaticString put the field width in the type, so each new width recompiled the DiskArrays/AbstractArray read path plus String/show.

Breaking: eltype(var) for char variables changes, and CommonDataFormat.StaticString is removed (nothing else in the org uses it).

Measurements (fresh process, min of 5)

scenario	main	this PR
using	0.063 s	0.066 s
one CDAWeb file: read data + labels, show labels	0.204 s	0.218 s
every char var in data/: Array, v[1], String., show	1.98 s	0.80 s
Array of every var in data/	0.33 s	0.15 s
precompile time / pkgimage	3.1 s / 9.8 MB	3.2 s / 9.7 MB

First call per new width: Array(label) 10–14 ms → 0, label[1] 29 ms → 0.

Runtime: Array of an 8-element label goes from 151 ns to 229 ns. Synthetic 1e6 × 16-byte records: 16 ms to build Strings vs 0.26 ms memcpy (56 MB vs 16 MB). That only matters for very large record-varying string variables.

The single-file case is unchanged because its time goes to Base compiling show for Matrix{String} (~140–190 ms). Both versions pay that.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/loading/variable.jl	92.30%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	79ebf46...	main / 79ebf46...
elx/full_load	24.6 ± 1.9 μs	24.6 ± 1.9 μs	1 ± 0.11
elx/sum_array	2.67 ± 1.3 μs	2.56 ± 0.84 μs	1.04 ± 0.6
elx/sum_lazy	2.75 ± 0.51 μs	2.71 ± 0.64 μs	1.02 ± 0.31
elx/sum_var_access	1.93 ± 0.99 μs	1.97 ± 1.2 μs	0.982 ± 0.78
elx/var_access	0.11 ± 0.01 μs	0.111 ± 0.011 μs	0.991 ± 0.13
epochs/DateTime_to_TT2000	0.0653 ± 0.00033 ms	0.0621 ± 0.0003 ms	1.05 ± 0.0074
epochs/Epoch16_parse	0.31 ± 0.01 ms	0.31 ± 0.01 ms	1 ± 0.047
epochs/Epoch16_vs_DateTime	2.92 ± 0.01 μs	2.92 ± 0.01 μs	1 ± 0.0048
epochs/TT2000_parse	0.364 ± 0.01 ms	0.357 ± 0.01 ms	1.02 ± 0.041
epochs/TT2000_string	5.76 ± 1.5 ms	5.61 ± 1.6 ms	1.03 ± 0.4
epochs/TT2000_to_DateTime	0.118 ± 0.00064 ms	0.112 ± 0.00047 ms	1.06 ± 0.0073
epochs/TT2000_vs_DateTime	0.0587 ± 1e-05 ms	0.0525 ± 2e-05 ms	1.12 ± 0.00047
mms/full_load	17.4 ± 1.9 μs	17.3 ± 1.9 μs	1.01 ± 0.15
mms/sum_array	0.0948 ± 0.0015 s	0.0952 ± 0.0021 s	0.996 ± 0.027
mms/sum_slice	3.49 ± 0.065 ms	3.49 ± 0.41 ms	1 ± 0.12
mms/sum_var_access	3.49 ± 0.055 ms	3.5 ± 0.4 ms	0.996 ± 0.12
mms/var_access	0.07 ± 0.01 μs	0.07 ± 0.001 μs	1 ± 0.14
time_to_load	0.167 ± 0.00084 s	0.167 ± 0.0022 s	0.999 ± 0.014

Memory benchmarks

	main	79ebf46...	main / 79ebf46...
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	0.056 k allocs: 5.77 kB	0.056 k allocs: 5.77 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	0.204 k allocs: 11.9 kB	0.984