separate is_standard from new property is_classification

[gkennos]

Pre-flight

• I searched existing issues and this has not been requested before.

Problem or motivation

Concept.is_standard and Concept.is_standard_expr() are named for standardness but test membership of StandardConceptFlag.values through frozenset({'S', 'C'}).

Classification concepts therefore satisfy a predicate called is_standard

@property
def is_standard(self) -> bool:
    value = self.standard_concept.strip() if self.standard_concept is not None else ""
    return bool(value) and value in StandardConceptFlag.values

@classmethod
def is_standard_expr(cls) -> sa.SQLColumnExpression[bool]:
    """SQL-side counterpart to :attr:`is_standard`, for use in query filters."""
    return normalised_flag_expr(cls.standard_concept).in_(StandardConceptFlag.values)

Proposed solution

Classification concepts are not valid mapping targets therefore they must be separable in downstream consumers

Replace ambiguous predicate with separate, correctly named ones

is_standard        / is_standard_expr          - 'S' only
is_classification  / is_classification_expr  - 'C' only

Important note:

negation must return the complement (~is_standard_expr() counts total − standard), and the Python and SQL halves must agree across None/''/' '/'S'/'C'/junk