[scheduler] EEVDF scheduler

[laveeshb]

Description

Document the EEVDF (Earliest Eligible Virtual Deadline First) scheduler:

• Why it replaced CFS internals
• Eligibility and deadline concepts
• Latency improvements
• Migration from CFS

Goal

Explain the modern scheduler design (kernel 6.6+).

Part of

Epic: Scheduler Fundamentals

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Parent: #141