docs: update ROADMAP.md to reflect v3.0.0 shipped and current state
Migration script: the early-exit check now verifies the dist track matches
the installed major version. A v2 node that had v3 dist track from a test
run now gets corrected rather than skipped.
ROADMAP.md: full update — v3.0.0 moved to Released, GitHub Pages apt repo
added as Released, v3.1.0 scope updated with #272 and multipath redesign,
stale pre-release housekeeping removed.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>