Simplify version guidance and align demo editors

[haoranyu]

Closes #62

Summary

• removes the CM5/CM6 switcher from the v2 homepage and demo headers and keeps quiet contextual links in each hero
• pins the exact v1 source merged through Move v1 version guidance out of the header #63 for the versioned Pages build
• matches the v2 CodeMirror 6 demo to the v1 editor paint while retaining CM6 folding and editing behavior
• adds computed-style parity for typography, gutter, active line, selection, cursor, and semantic tokens
• exercises header layout and navigation at desktop, tablet, and 320px mobile widths

Verification

• npm run test:all
• npm run test:coverage — 148/148 tests; core src statements 98.66%
• exact versioned site assembly pinned to v1 commit 149c86f954d0798642101bc1000e770a57e1ee18
• versioned browser smoke with external assets blocked — both real CM5 and CM6 demos captured, loaded, and replayed successfully
• independent Standards review: no remaining findings
• independent Move version guidance out of the site headers #62 Spec review: no remaining findings

The v1 half was reviewed and squash-merged separately in #63 before this main-branch change.