fix: RLE decompression for compressed variable records

[Beforerr]

decompress_bytes! had an empty RLE branch: RLE-compressed CVVRs silently
left dest uninitialized. Route RLE through _rle_decompress (output size
is known from the read request) and throw ArgumentError for unsupported
codecs (Huffman/AHUFF) instead of @Assert.

No CDF writer in the ecosystem emits variable-level RLE (cdflib/pycdf
fixtures are gzip-only, which is how this went untested), so tests build
CVVRs by hand.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	9f2791e...	main / 9f2791e...
elx/full_load	0.12 ± 0.0043 ms	0.117 ± 0.0042 ms	1.03 ± 0.052
elx/sum_array	3.38 ± 0.36 μs	3.48 ± 0.39 μs	0.971 ± 0.15
elx/sum_lazy	3.33 ± 0.42 μs	3.35 ± 0.45 μs	0.991 ± 0.18
elx/sum_var_access	3.75 ± 0.39 μs	3.82 ± 0.5 μs	0.982 ± 0.16
elx/var_access	0.31 ± 0.001 μs	0.31 ± 0.011 μs	1 ± 0.036
mms/full_load	27.1 ± 2.4 μs	27.1 ± 2.6 μs	1 ± 0.13
mms/sum_array	0.0956 ± 0.0027 s	0.0957 ± 0.0029 s	0.999 ± 0.041
mms/sum_slice	3.59 ± 0.071 ms	3.59 ± 0.46 ms	0.999 ± 0.13
mms/sum_var_access	3.58 ± 0.053 ms	3.59 ± 0.26 ms	0.998 ± 0.074
mms/var_access	0.11 ± 0.01 μs	0.11 ± 0 μs	1 ± 0.091
time_to_load	0.144 ± 0.00072 s	0.146 ± 0.0013 s	0.989 ± 0.01

Memory benchmarks

	main	9f2791e...	main / 9f2791e...
elx/full_load	3.73 k allocs: 0.167 MB	3.73 k allocs: 0.167 MB	1
elx/sum_array	5 allocs: 27.7 kB	5 allocs: 27.7 kB	1
elx/sum_lazy	5 allocs: 27.7 kB	5 allocs: 27.7 kB	1
elx/sum_var_access	8 allocs: 28.1 kB	8 allocs: 28.1 kB	1
elx/var_access	2 allocs: 0.328 kB	2 allocs: 0.328 kB	1
mms/full_load	0.243 k allocs: 14.8 kB	0.243 k allocs: 14.8 kB	1
mms/sum_array	0.524 k allocs: 31.6 MB	0.524 k allocs: 31.6 MB	1
mms/sum_slice	0.045 k allocs: 1.29 MB	0.045 k allocs: 1.29 MB	1
mms/sum_var_access	0.045 k allocs: 1.29 MB	0.045 k allocs: 1.29 MB	1
mms/var_access	2 allocs: 0.328 kB	2 allocs: 0.328 kB	1
time_to_load	0.145 k allocs: 11 kB	0.149 k allocs: 11.2 kB	0.983

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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