Fix h5sign default algorithm, documentation accuracy, and stale comments

- Change h5sign default hash algorithm from SHA-256 to SHA-512 to match
  the 4096-bit RSA key recommendation (consistent security level)
- Fix hardcoded "Computing SHA-256 hash..." verbose message to use the
  actual algorithm name via EVP_MD_get0_name()
- Update h5sign help text: default, examples, and key generation command
  all updated to reflect SHA-512 default and 4096-bit keys
- Fix PLUGIN_SIGNATURE_README.md inaccuracies:
  - Correct fabricated output example to match actual h5sign output
    (field names, format, arithmetic, security reminders block)
  - Fix HDF5_DEBUG=PL -> HDF5_DEBUG=pl (case-sensitive package name)
  - Fix magic number byte order: 48 44 46 35 -> 35 46 44 48 (little-endian)
  - Fix fabricated composite error message heading
  - Add missing file size component to cache key description
  - Clarify keystore priority: env var wins exclusively, not a fallback chain
  - Add note that HDF5_REQUIRE_SIGNED_PLUGINS=ON requires PLUGIN_KEYSTORE_DIR
  - Correct signature size range from 256-512 to 256-1024 bytes
  - Fix h5signverifytest: clarify it is an internal test harness
  - Update performance numbers to realistic values with explanatory note
  - Fix key size wording: "4096-bit recommended, 2048-bit minimum"
  - Add h5cc availability caveat
  - Fix all markdown lint warnings (MD022/031/032/034/040/060/024)
- Fix stale (default) comment on H5PL_SIG_ALGO_SHA256 in H5PLsig.h;
  add (default) to H5PL_SIG_ALGO_SHA512

Author: brtnfld