Score explanation renderer: format DimensionScore.explanation into Markdown/HTML

[emtcmca]

v1.0 roadmap candidate, and a genuinely good first issue — self-contained, no scoring logic involved.

What: Every dimension already emits an explanation string. This feature adds a renderer that turns a full scorecard into structured Markdown (and optionally HTML) for display in dashboards, logs, or PR-style review UIs.

Sketch:

renderScorecard(scorecard, { format: 'markdown' }) // => string

Constraints:

• Zero runtime dependencies — string templating only, no markdown/HTML libraries
• Pure function of the scorecard: same scorecard in, same string out
• Must include: total, label, recommended action + reason, per-dimension raw/max + explanation, warnings with codes

Start by reading src/types.ts (ConfidenceScorecard) — the renderer only consumes the public type.