docs: split verification and browser roadmaps

[hongnoul]

Summary

• replace the combined roadmap with a portfolio index for one shared platform and two products
• add dedicated AI verification, tiling-WM browser, and shared platform roadmaps
• define dependency direction, promotion gates, and incremental source-crate boundaries
• align the vision, contributor policy, guides, benchmark links, and website navigation
• correct stale display-free and trusted-input roadmap status while preserving every named work item

Validation

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• all changed local Markdown links resolve
• all 44 named items from the previous roadmap remain represented
• independent read-only documentation review completed