docs: strengthen presentation deck

[Maokami]

Summary

• rewrite the Verso slide deck into a clearer 10–15 minute presentation arc
• make the layered architecture and the trace-vs-collecting split much easier to grasp
• promote the Interval x0 = 5 collecting result to a headline slide

Validation

• cd site && lake build
• cd site && lake exe slides

Notes

• this is the first content pass on issue Make the Verso slide deck presentation-grade #117
• the deck still uses the current VersoManual-based rendering model
• future visual upgrades can build on this stronger narrative structure

[gemini-code-assist]

Code Review

This pull request updates the absinterp slide deck in site/AbsinterpSlides/Intro.lean. The content has been significantly restructured to better explain the project's goals, its layered architecture, and the distinction between the framework core and the IMP adapter. Key additions include an ASCII diagram of the architecture, a detailed section on the x0 = 5 flagship case study, and a clearer definition of the upstreaming boundary for CSLib. I have no feedback to provide.