Publish the public roadmap surface

What ships in this project

• File-based roadmap corpus at src/content/roadmap/ mirroring the Journal and Register patterns.
• Public index at /roadmap with status grouping and aggregate credit display (posted total, founder credits accrued).
• Single project page at /roadmap/[slug] with header chips for status, credit value, milestone, and accrued founder credits.
• Cross-link token P-NNN recognized by the existing remark plugin.
• Reactions mounted on each project page, reusing the same primitive as Journal and Register entries.

What this project does not ship

• No claim, submission, or acceptance flow. Contributors cannot yet earn credits through this surface — that's blocked on G-038.
• No founder-time logging UI; the founder_credits field is set manually in frontmatter for now.
• No comments / Q&A on projects; that's blocked on G-035.

Why this is its own first project

Entry 016 commits founder time to the same denomination as contributor time. The roadmap-publishing project itself is the most direct example of that principle in action: the work to build the roadmap surface is the first item on the roadmap, with founder credits visible.