Task: Commission subject_area onto standard codegen

Carried forward from Sprint 23's Task: Commission catalog/change_reason/change_reason_category/data_domain onto standard codegen, which fully commissioned four of the five entities in its original scope and deliberately stopped short of subject_area because its compound natural key (name, domain_name) doesn't fit lookup_entity's (now domain_entity's) single-column primary key shape. Commission subject_area onto the standard codegen stack once compound natural-key support lands (52FE40FA, in Codegen infrastructure follow-ups from DQ commissioning): author its .org model with the compound key properly expressed, regenerate core/ repository/service/handler/protocol, retire the hand-written equivalent, swap the SQL table over for real.

• subject_area has a codegen .org model under projects/ores.dq/modeling/ with its compound natural key (name, domain_name) properly expressed as the primary key, not descoped to a plain Column.
• Generated core/repository/service/handler/protocol code replaces the hand-written equivalent; SQL table swapped over for real.
• Build and tests green.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.