fix(tools): h5rs dump shows NUL padding in nested strings, like h5dump

A null-padded fixed string inside a compound or an array member printed
trimmed ("" for three NULs, "a" for "a\0b"), where h5dump prints every
byte ("\000\000\000", "a\000b"); only top-level strings were shown in
full. DATA blocks now render elements through one function that keeps
the padding at any depth.

The README now lists the remaining known differences from h5dump:
nested compounds print inline, and long double values are printed as
errors (exit 1) with the datatype as an H5T_FLOAT block.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -123,8 +123,10 @@
   - `h5rs dump [--json] [-A] [-p] [-d PATH] FILE` prints DDL text that is
     byte-identical to h5dump 1.14.6's on the test files (all layouts and
     chunk indexes, v1/v2 groups, compound, enum, strings, links, named
-    types, attributes), or JSON in the HDF Group's hdf5-json layout (schema
-    in the crate README).
+    types, attributes; null-padded strings show their NULs at any depth),
+    or JSON in the HDF Group's hdf5-json layout (schema in the crate
+    README). Nested compounds print inline and `long double` values as
+    errors (exit 1); both are listed in the README.
   - `h5rs stat FILE` reports h5stat's object, link, rank, layout, filter,
     attribute, raw-data and file-size figures (equal to h5stat's on the test
     files); metadata space is one figure, not broken down.