Type Meta.fields and local_fields as ColumnField

[davegaeddert]

Follow-up to #103.

Meta.fields, concrete_fields, and local_concrete_fields were typed ImmutableList[Field] even though the only Field subclasses that aren't ColumnField are the abstract RelatedField and ManyToManyField, and M2M fields are routed to local_many_to_many in add_field. So every element really is a ColumnField, and #103 had to add an assert isinstance(f, ColumnField) inside clean_fields to read column attributes off it, next to a pre-existing one in Model.__init__.

• Type local_fields, fields, concrete_fields, and local_concrete_fields as ColumnField.
• Enforce the split once in add_field (raise TypeError for anything that is neither) instead of re-checking in every loop.
• fields is now just an immutable copy of local_fields; the "generic relation" filter it carried from Django was dead code since no such field class exists here.
• Drop the two isinstance(…, ColumnField) asserts in base.py and the equivalent skip in the convergence column comparison.
• Widen QuerySet._update and UpdateQuery.add_update_fields to Sequence so a list[tuple[ColumnField, Any]] passes without an invariance error.

Type ignores 148 → 146, asserts 176 → 173.

Test plan

• ./scripts/type-validate 26/26
• ./scripts/test plain-postgres

[pullapprove5]

PASS: 1 review scope passed, 1 agent approved

	Scope	Progress
✅	code	1/1

✅ codex approved · 1 dismissed

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Next steps:

• @davegaeddert: Merge or close