Add docs/ROADMAP.md and pinned status tracker #10

Problem

The phase plan for rustmanifest is currently scattered across:

• The Phase 0 issue (Phase 0: Foundation — workspace, CI, release pipeline, schemas, governance #2) — initial outline only.
• Per-PR "Out of scope (deferred)" sections in 2 #3, 5 #6, 4 #7, 8 #9.
• This author's running conversation.

A first-time visitor cannot read a single canonical document explaining what the project is, what is done, and what is next. For a public, production-grade project this is a gap.

Fix

Two pieces, one PR for the document and one follow-up gh-api call for the pin:

1. docs/ROADMAP.md — canonical phase plan, versioned with the code. Per-phase blocks: goal, scope, deliverables, status (done / in progress / planned), linked issues. Top of the file shows current phase at a glance.
2. Pinned tracker issue — opened after the doc lands, links to docs/ROADMAP.md, summarises current status. Pinned via gh api graphql ... pinIssue. The tracker stays open and updated until the project ships v1.0.

Out of scope

• GitHub Project board (kanban) — overhead vs payoff is wrong for a solo maintainer.
• Per-phase milestone objects on GitHub — adds little beyond per-issue labels, deferred unless multi-maintainer.

Acceptance criteria

• docs/ROADMAP.md committed and linked from README.md.
• reuse lint 100%.
• CI green.
• After merge: tracker issue opened with a link to docs/ROADMAP.md, pinned to the repository.
• Each future phase PR description starts with Updates docs/ROADMAP.md status to <new state> (or contains the actual edit to the file).