Add tasks with dependency chains section to SDL3 GPU migration plan

[Copilot]

User requested GitHub Issues for migration tasks with dependency chains. Unable to create GitHub Issues directly due to tooling limitations.

Instead, added a structured "Tasks with Dependency Chains" section to docs/sdl3_gpu_migration_plan.md documenting:

• Dependency table - All 16 tasks (Task 0-15) with explicit dependencies and phase mappings
• ASCII dependency graph - Visual representation of task relationships
• Critical path - Identifies longest dependency chain: Task 0 → 1 → 3 → 4 → 5 → 6 → 7/8/11/12/13 → 14 → 15
• Parallelization opportunities - Groups of tasks that can execute concurrently

Note: If actual GitHub Issues are required, the dependency information in this section can be used to create them manually or via gh issue create commands.

Original prompt

On sdl3_gpu_migration_plan.md, Create tasks with dependency chanis for each of these steps

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