fix(graphql): UUID/DateTime/Date/Time as first-class scalars (6.1.0)

[allmonday]

Port of nexusx #104 / #92 / #105. uuid.UUID, datetime, date, time all fell back to String — the scalar map only knew int/str/float/bool and map_scalar_type used fragile substring matching.

Root cause (verified on master)

• id: UUID → id: String!, created_at: datetime → String!, d: date → String!, t: time → String!
• Introspection advertised only Int/Float/String/Boolean/ID — UUID/DateTime/Date/Time missing, so GraphiQL couldn't discover them
• list[UUID] / list[datetime] / single @query/@mutation params arrived as raw str → crashes SQLAlchemy/SQLModel on UUID/datetime column binding

Fix

• type_mapping.py: subclass-matched UUID/datetime/time/date (datetime ordered before date) → SDL + introspection field type-refs render UUID!/DateTime!/Date!/Time!
• type_registry.py: register the 4 scalars in SCALAR_TYPES; derive TypeRegistry._scalars from it (kills the two-list drift)
• introspection_generator.py: advertise every SCALAR_TYPES entry (was a hardcoded 5-name list)
• executor.py: _convert_scalar_value / _coerce_scalar coerce these args from string to the real object, recursing through list[T], Optional[T], Optional[list[T]]

Verification

• New repros: list[UUID]→[UUID], list[datetime]→[datetime], single UUID/date/time→real objects, Optional[list[UUID]]→[UUID] ✅
• tests/graphql/: 252 passed (was 248, +4). Tightened test_datetime_support (pinned scalar names, real UUID type, dropped the permissive String OR) + added list[UUID]/list[datetime]/single-scalar arg-conversion regressions + introspection-advertising check
• tests/ + demo/: 905 passed, 1 skipped
• ruff: clean

Not bugs (verified — not ported)

• Self-referential types → PR handles fine (no RecursionError, unlike nexusx rename loader_filters #91)
• has_more off-by-one (nexusx add schema_config #86) → PR doesn't compute it (user page_loader owns it)
• SDL limit/offset on paginated fields (remove ensure_type #98) → PR already has it

Version

6.1.0 (minor) — SDL surface change: fields/args of these types change String → UUID/DateTime/Date/Time. Transport unchanged. SDL consumers (codegen/Apollo) should regenerate. Decimal unchanged. Bumps pyproject.toml + uv.lock + changelog; publish via v6.1.0 tag after merge.

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