feat(mcp): compare_viewports tool

[aram-devdocs]

Summary

Delta layout between two viewports — elements missing / size
change > threshold / ordering change.

Crate / area

plumb-mcp — rmcp-based stdio MCP server

Phase / batch

Part of #65 → Batch 7C (MCP V1 — new tools + HTTP transport).

Effort: M

PRD references

• PRD §14.2

Acceptance criteria

• Tool returns structured delta per node.
• Handles the common cases (hidden at mobile, reordered, reflowed).

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-mcp/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue