feat: add bounded product-excellence exploration

[Ldsystem]

Summary

• add one compact evidence-routed excellence applicability pass to specification authoring
• keep proposals non-authoritative unless explicitly accepted and projected to stable IDs
• add pressure scenarios and deterministic contract coverage for no-opportunity, deferred, accepted, silent-scope, checklist, and safety cases

Validation

• 68 focused tests passed
• orchestration doctor: ok
• executor-result validation: passed
• independent task review: accept, no findings

Linear: WOR-62