[VSJoin-Exec] Tracker: Ch3 idea-code-Ch4/Ch5 alignment (new track)

[ZeroJustMe]

Goal

Create a new execution track (independent from #112-#123) for Chapter 3 idea-to-code and Chapter 4/5 alignment.

Deliverables

• Mechanism I closure
• Mechanism II closure
• Mechanism III closure
• Chapter 4/5 rewrite alignment
• Experiment matrix + ablations
• Final quality gate

Exit Criteria

• One-to-one mapping: claim -> mechanism -> metric -> figure
• Bounded conclusions under evaluated workloads