feat: deliver Spreadsheet Auditor dogfood walking skeleton

[BeforeLights]

Summary

• Deliver a testable three-platform dogfood vertical without restricted vendor APIs.
• Add Desktop approved-folder local scanning and guarded folder-grant IPC.
• Add content-free IAE local artifact registration, clean-scan admission, evidence resolution, and OpenAPI contracts.
• Add deterministic Spreadsheet Auditor engine action, run-admission API, Web review page, and cross-runtime dogfood readiness gate.
• Preserve the existing DataBreeze brand assets.

Verification

• corepack pnpm repo:check
• corepack pnpm repo:build
• API 476/476; Engine 115/115 Python plus runtime checks; Desktop 28/28; Web 29/29 plus bundle budget.
• 38 commits, below the review ceiling.

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