visual-compare: support comparing against previous run baselines

[chubes4]

Summary

wordpress.visual-compare should support comparing a current result against a previous visual comparison run or declared baseline artifact.

Problem

Callers often need to know whether a current change improved, regressed, or stayed neutral relative to earlier evidence. Today that comparison requires external bookkeeping and custom result aggregation.

Proposed shape

• Accept a previous visual-diff.json or aggregate visual comparison artifact as a baseline.
• Emit deltas for mismatch ratio, mismatch pixels, dimensions, and status.
• Support both single-comparison and matrix comparison outputs.
• Keep policy decisions outside the command; report evidence only.

Acceptance criteria

• A visual comparison run can report deltas against a previous run.
• The output clearly distinguishes current source/candidate comparison from baseline delta comparison.
• The command does not enforce pass/fail policy beyond command execution success.