Task: Compound-key is_uuid gating drops trailing columns in Qt/service templates

cpp_protocol.hpp.mustache and cpp_service.cpp.mustache gate their delete/history/save is_uuid branch on primary_key.is_uuid alone, which reflects only the first key column. A future compound key whose leading column is UUID would silently drop every subsequent key column from these structs/validations. Not exercised today (subject_area's key is two text columns). Raised in PR #1691 review round 1. Fix: make the is_uuid branches loop primary_key.columns like cpp_domain_type_mapper.cpp.mustache already does.

(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.)

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