refactor: R{FST}(buffer, offset) record constructor style

[Beforerr]

Move the field-size type from a trailing positional argument into the
type parameter for the records that already carry it (CDR, GDR, VDR,
rVDR, VXR, ADR). CPR/CCR/CVVR keep the positional form: their structs
are not parameterized by FST — there it is a parsing parameter, not
type identity. No behavior change; trim-equivalent (where-param either
way).

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	b1d10fd...	main / b1d10fd...
elx/full_load	0.0387 ± 0.0021 ms	0.0385 ± 0.0021 ms	1.01 ± 0.077
elx/sum_array	2.83 ± 0.65 μs	2.98 ± 0.4 μs	0.95 ± 0.25
elx/sum_lazy	3.02 ± 1.1 μs	3.02 ± 0.61 μs	1 ± 0.41
elx/sum_var_access	3.4 ± 0.57 μs	3.39 ± 0.47 μs	1 ± 0.22
elx/var_access	0.31 ± 0.01 μs	0.31 ± 0.001 μs	1 ± 0.032
mms/full_load	22.2 ± 2 μs	22.3 ± 2.2 μs	0.996 ± 0.13
mms/sum_array	0.0953 ± 0.013 s	0.0953 ± 0.0026 s	1 ± 0.14
mms/sum_slice	3.58 ± 0.057 ms	3.58 ± 0.08 ms	0.999 ± 0.027
mms/sum_var_access	3.57 ± 0.047 ms	3.57 ± 0.06 ms	1 ± 0.021
mms/var_access	0.111 ± 0.01 μs	0.11 ± 0 μs	1.01 ± 0.091
time_to_load	0.135 ± 0.0013 s	0.134 ± 0.0021 s	1 ± 0.018

Memory benchmarks

	main	b1d10fd...	main / b1d10fd...
elx/full_load	0.246 k allocs: 27.9 kB	0.246 k allocs: 27.9 kB	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	0.071 k allocs: 7.2 kB	0.071 k allocs: 7.2 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	0.145 k allocs: 11 kB	1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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