Expose DSID product specification subfields (PRSP, PSDN, PRED, PROF)

[philliphoff]

Fixes #17. This is needed for inland ENC support in philliphoff/EncDotNet.S100#608.

Changes

• S57DataSetIdentification: four new properties, added after the existing ones:
  • ProductSpecification (PRSP, int). The docs note that S-57 §7.3.1.1 enumerates 1 = ENC and 2 = ODD, and that inland ENC producers declare 10.
  • ProductSpecificationDescription (PSDN, string)
  • ProductSpecificationEdition (PRED, string)
  • ApplicationProfile (PROF, int)
• S57DocumentReader: populates the new properties with the same TryGetSubfield pattern used for AGEN / COMT, defaulting to 0 / empty when a subfield is absent.
  • Handling of PRSP and PROF: their binary form is b11, but their ASCII form is a mnemonic (A(3) ENC/ODD, A(2) EN/ER/DD).
  • Reading them with byte, as INTU is read, would throw on an ASCII cell.
  • So they are read as strings (the field reader turns a binary value into its decimal text) and mapped to the numeric code. Digits such as 10 are parsed directly, and unknown values become 0.
• Doc fix: ProducingAgency now says (AGEN) instead of (PRSP).
• README: not changed, because it doesn't list DSID members.

Tests

New tests in S57DocumentReaderTests use synthetic data only:

• Binary DSID: PRSP / PSDN / PRED / PROF values for NOAA (1, "2.0") and inland (10, "2.4") are read correctly.
• ASCII-form DSID: the mnemonics ENC/ODD/EN/ER/DD and a numeric 10 map to the right codes.
• DSID definition without the four subfields: the properties default to 0 / empty.

I ran the tests with dotnet test tests/EndDotNet.UnitTests in the repo on macOS. The NU1605 restore error did not occur, so the scratch project was not needed. All 492 tests pass.

Both format gates are clean:

• dotnet format whitespace EncDotNet.slnx --verify-no-changes
• dotnet format style EncDotNet.slnx --diagnostics IDE0005 --severity warn --verify-no-changes