A tag shorthand using a named handle (`!handle!suffix`) must be declared by a
`%TAG` directive in the document that uses it; the handle does not carry across
documents (YAML 1.2). The parser was permissive but silent here, so an
undeclared handle passed unnoticed.
- Scope tag handles per document: a `%TAG` handle now applies only to the
document it precedes (merged onto the defaults at the document start, cleared
at the boundary) instead of accumulating across the whole stream. A directive
repeated for each document keeps working, as the suite requires.
- Detect the named-handle form before the primary-handle fallback (which
previously masked `!prefix!x` as a local tag) and record an undeclared handle.
The parse stays permissive: it still yields a value, and
`PureYAML.validationReport` surfaces the issue as an `undefinedTagHandle`
error. The check reads the scanner's tokens, so it is exact: a `!x!` inside a
scalar or comment is never flagged.
Extracts `Scanner.State` and the tag helpers into their own files to keep each
source file within the 400-line limit. Adds focused tests; the full suite and
strict lint stay green.
