[philliphoff]

Summary

The "Smaller, nice to have" items of #49. Items 1–3 are in a separate PR.

Typed codes. Each enum value is the exact S-57 code, so callers can cast to and from the existing int properties, and codes an enum doesn't name still cast through as numbers. The int properties and the records' ToString output are unchanged; each property's XML docs now name its enum.

Subfield	Enum
EXPP	S57ExchangePurpose (New 1, Revision 2)
PRSP	S57ProductSpecification (Enc 1, DataDictionary 2, InlandEnc 10)
PROF	S57ApplicationProfile (EncNew 1, EncRevision 2, DataDictionary 3)
DSTR	S57DataStructure (1–4, TopologyNotRelevant 255)
HDAT	S57HorizontalDatum (all 131 HORDAT values; WGS84 = 2)
VDAT/SDAT	S57VerticalDatum (all 30 VERDAT values)
DUNI/HUNI/PUNI	S57DepthUnits, S57HeightUnits, S57PositionalAccuracyUnits
COUN	S57CoordinateUnits

S57ProductSpecification is there for writers. S57ProductKind is for reading and can't be cast back to a code: its InlandEnc is 3, not 10. The AML codes 51–57 stay with AmlProduct, whose values are already PRSP codes.

Lexical levels.

• S57LexicalLevel names the levels: Ascii 0, Latin1 1, Ucs2 2.
• S57LexicalLevels.Lowest(IEnumerable<string?>) returns the lowest level that can encode every string. An empty set gives Ascii.
• S57LexicalLevels.CanEncode(string?, S57LexicalLevel) checks a single string.
• S57LexicalLevels.WithLowestLevels(S57Document) is an opt-in way to set AALL/NALL from a document's ATTF/NATF values. It throws if an ATTF value needs UCS-2, which ATTF doesn't allow. The writer's default output is unchanged.
• Characters outside the Basic Multilingual Plane, and unpaired surrogates, can't be encoded at any level, so Lowest throws ArgumentException.
• These methods use the writer's own encoder (S57FieldEncoder), so the two can't disagree. Its surrogate check moved into a shared method; the bytes written are the same.

This replaces EncDotNet.S100's S101ToS57LexicalLevels.

Docs. In docs/writing-s57-charts.md, the example uses the enums, there's a subfield-to-enum table, and WithLowestLevels is explained.

Left open:

• S57ApplicationProfile names only the S-57 codes 1–3. The AML profile codes 4–20 cast through as unnamed values.
• There's no TryLowest variant for non-BMP text.

Standards alignment

• ISO/IEC 8211 (iso8211)
• S-57 records, topology, updates or exchange sets (s57-enc)
• Legacy chart viewer, maintenance only (chart-viewer)
• N/A: build, CI, docs or tooling only

Standard section references cited in code or docs: S-57 Part 3 §7.3.1.1 (DSID/DSSI), §7.3.2.1 (DSPM) and clause 2.4 (lexical levels); Appendix A attributes HORDAT and VERDAT.

Tests

• Added or updated xUnit tests under tests/. They cover:
  • every enum value against the spec, including no gaps in HORDAT/VERDAT
  • lexical levels for ASCII, Latin-1, BMP, non-BMP, unpaired surrogates and an empty set
  • writer round trips, including WithLowestLevels reproducing the golden cell byte for byte
• Test input is built in memory; no real ENC cells are committed
• dotnet test --configuration Release passes: 1036 unit tests and 14 performance tests. It also passes merged with the other EncDotNet.S57: attribute catalogue, object codes 161+ and long-name helpers #49 PR.

Documentation

• Updated the affected package's src/<project>/README.md
• Updated guides under docs/ if user-facing behavior changed
• New public APIs have XML doc comments
• docfx docfx.json --warningsAsErrors passes, if docs changed

Dependencies

• No new NuGet dependencies

Breaking changes

None. All the types are new, and existing properties and writer output are unchanged. dotnet pack package validation against 0.8.1 passes.

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