Add project roadmap and architecture docs

docs/ARCHITECTURE.md

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+# Architecture
+
+Serato Doctor is designed as a DJ library inspection, repair, and migration platform.
+
+## Design Principles
+
+1. Read-only by default.
+2. Every repair must support preview/dry-run.
+3. Every repair must create a backup or rollback path.
+4. Application-specific logic lives in engines.
+5. Core matching and scanning logic should be application-agnostic.

docs/case-studies/onedrive-mac-migration.md

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+# Case Study: OneDrive Mac Migration
+
+## Scenario
+
+A large Serato DJ library was migrated from an older Mac to a newer Mac using OneDrive.
+
+## Symptoms
+
+- OneDrive client stuck syncing
+- Duplicate OneDrive folders
+- Thousands of files renamed with trailing ` 2`
+- Serato reported many tracks as missing
+- Some files existed on disk but still appeared orange in Serato
+
+## Findings
+
+- OneDrive sync state was rebuilt successfully
+- Thousands of orphaned filename conflicts were repaired
+- Some Serato references were stale database objects, not missing files
+- Smart/dynamic crates should be classified separately from static user crates
+
+## Lessons
+
+- Filesystem health and Serato database health are separate problems
+- Smart crates should not be treated the same as static crates
+- Repair tools must be read-only by default and generate a plan before changing anything