feat: experimental electrobun desktop migration

[boggedbrush]

Summary

This PR publishes the current electrobun desktop migration work as a highly experimental spike.

The branch includes:

• a new apps/electrobun app and Bun/Electrobun desktop entrypoint
• desktop runtime plumbing shared across desktop and web surfaces
• benchmark, correctness, and reporting scripts plus migration assessment docs
• server, contracts, and web changes needed to exercise the experimental runtime path

Status

This is intentionally an experimental branch PR.

It may be incomplete, partially implemented, or subject to substantial redesign before any merge decision. The goal is to preserve the current state for review and iteration, not to present this as production-ready work.

Why

The change set captures an early desktop migration spike so the approach, constraints, and measurements can be reviewed in one place.

Validation

• bun fmt
• bun lint
• bun typecheck

Notes

• An untracked local file named nul was left out of the commit because it appears to be accidental shell output rather than part of the feature work.