Phase A: typed-header value-type scaffolding (Token, Host, Origin, ...)

[nielsenko]

Sub-issue of #113 covering Phase A of the typed-header overhaul: the shared value-type scaffolding that the per-header fixes will build on.

See the audit comment on #113 for the full plan and the per-header checklist.

Scope

One commit per primitive, in dependency order. Each commit ships with unit tests and no behavioral changes to existing typed headers.

• A1. Token abstract interface class. tchar validation per RFC 9110 5.6.2, ASCII case-insensitive equality, sf-token serialization helper.
• A2. HeaderScanner primitive. Shared tokenizer with tchar reader, quoted-string reader (with quoted-pair), OWS skip, comma-list iterator, semicolon-param iterator. Replaces the splitTrimAndFilterUnique / split(';q=') / split('=') family that header parsers reach for today. Closes Improve HTTP header parsing to handle quoted strings with delimiters #102.
• A3. DeltaSeconds. Non-negative integer with validation, formatting, and a digit-only parse helper.
• A4. Host. uri-host plus optional explicit port (port absent is distinct from port=default). Factory from Uri that respects hasPort.
• A5. Origin. Built on Host + scheme. Explicit null sentinel as a distinct case. Strict construction (no path/query/fragment).
• A6. LanguageTag (BCP 47). Extlang, script, variant, extension, private-use, grandfathered. Canonical casing on output.
• A7. ETagValue refactor. Tighten the existing type: validate etagc charset, stop stripping internal ", weak flag preserved.
• A8. ParameterValue (token / quoted-string choice). Uses A1 for the token form and A2 for the quoted-string form.

Out of scope here

• MailboxAddress (RFC 5322) is deferred. from_header.dart will stay String-typed for now and the singleton-vs-list bug gets fixed without introducing a structured mailbox type.
• All per-header fixes. Those land as separate commits in Phase B and onward, ticking boxes on the Overhaul typed header parsers #113 audit comment.

Notes for reviewers

• These primitives are zero-cost where possible (Dart extension types) and abstract interface class for Token so closed-enum headers can implement Token without losing their enum nature.
• Each primitive is independently mergeable. Order above is only the recommended landing order; later items don't strictly block earlier ones except where noted.