refactor: clean up Rust migration leftovers

[killswitch-GUI]

Summary

• audit all 49 Rust crate test boundaries and document the retained unit, integration, live, platform, and end-to-end evidence
• remove the retired nested research search adapter, Python-era theme importer, unused REPL history aliases, and their obsolete tests
• replace stale internal reconstruction notes with current Rust practices, test strategy, feature ownership, and ADRs; move optional SearXNG and OpenSearch stacks under examples

Breaking changes

• research search now requires top-level search profiles and a research role mapping
• custom themes must use schema version 1

Both changes are documented in the changelog and upgrade guide for this alpha release line.

Validation

• cargo xtask check rust
• cargo xtask check docs
• cargo xtask pr --base origin/main
• focused runtime, presentation, and research smoke tests
• Docker Compose configuration validation for both relocated service examples

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