WrzDJSet: Serato .crate + Engine DJ DB write

[thewrz]

Context

Native write paths for Serato and Engine DJ, replacing the XML-import workaround. Exec summary §8 (v3).

Scope

• Serato .crate file write (binary format — serato-tags / SSL-API references in repo docs)
• Engine DJ database write
• Delivered through Bridge (local file-system access) with conflict-safe write strategy
• Platform picker badges updated

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Work Estimate

• Estimated effort: 56 hours (~7 workdays).
• Planning phase: v3 Scale.
• Planning milestone: v3-Scale.
• Workplan target: 2026-09-10 milestone due date.
• Basis: High risk due to local filesystem writes, Serato crate format handling, Engine DJ DB writes, and conflict-safe Bridge delivery.