feat(strings): add Semantics.Strings.Identifiers package (Phase 0)

[matt-edmondson]

Summary

Adds a new ktsu.Semantics.Strings.Identifiers package — the first batteries-included concrete semantic-string types on top of the Semantics.Strings framework, which until now shipped only the validation framework with zero concrete types. This is Phase 0 of the semantic-domains roadmap (docs/roadmap-semantic-domains.md), scoped to the identifier subset.

Six validated identifier types, each a sealed record : SemanticString<T> with one package-local validation attribute:

Type	Validation	Normalisation
Uuid	RFC 4122 canonical 8-4-4-4-12 hex	lowercased, braces stripped
Ulid	26-char Crockford base32, timestamp-bounded first char	uppercased
Iban	structure + ISO 7064 mod-97-10 checksum	whitespace stripped, uppercased
Isbn	ISBN-10 (mod-11, X) or ISBN-13 (mod-10) check digit	hyphens/spaces stripped, uppercased
CreditCardNumber	13–19 digits + Luhn	spaces/hyphens stripped
JwtToken	3 base64url segments; header+payload decode to JSON objects	none (opaque)

Design decisions

• Framework stays generic. The six new check-digit/format validators live in the Identifiers package (subclassing the framework's public NativeSemanticStringValidationAttribute / ValidationAdapter); nothing domain-specific leaks into Semantics.Strings.
• Empty is never valid. Unlike some framework attributes (e.g. IsEmailAddress) which pass on empty to compose with a length rule, identifier attributes reject "" — Uuid.Create("") throws ArgumentException.
• Pragmatic validation with documented limits (matches the IsEmailAddress precedent): real check-digit maths where a checksum exists; documented non-goals (e.g. JWT signature is not verified; IBAN per-country BBAN tables not enforced; no BIN/PCI for card numbers).
• v1 is validation + type-safety only — no parsed accessors (ToGuid(), JWT claim decode); those are a clean follow-up (YAGNI).
• Out of scope (deferred): encodings/notations (Base64, HexString, SemVer, HexColor) → future Semantics.Strings.Formats; web types (Email, Url, …) → future Semantics.Strings.Web.

Testing

• 40 new MSTest tests across the six types (valid canonical, canonicalisation, bad-checksum, structural-invalid, empty-throws, plus an As<T> round-trip). Full suite: 944/944 passing, solution build clean (0 warnings across all five TFMs net10.0–net8.0 + netstandard2.0/2.1).
• Every checksum (Luhn, ISBN-10/13, IBAN mod-97) and the JWT base64url/JSON-object logic was independently hand-verified during review; the netstandard2.0 API surface was checked (JsonDocument.Parse(byte[]), Convert.FromBase64String, Regex timeouts all available).

Docs

• Spec: docs/superpowers/specs/2026-07-01-concrete-semantic-strings-design.md
• Plan: docs/superpowers/plans/2026-07-01-semantics-strings-identifiers.md
• README "Batteries-included identifiers" subsection + CLAUDE.md project-layout row.

Note on versioning

New additive package — the merge commit should carry a [minor] tag per the ktsu versioning convention.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
93.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud