chore(spec): auto-polish subplot-mosaic

[claude]

Automated spec polish from daily-regen pre-flight.

Spec: subplot-mosaic

What changed

• Removed non-canonical plot_type tags (subplot, multi-panel) — only mosaic is in the canonical vocabulary table
• Removed dashboard from features — it's an application use-case, not a visualization property
• Updated updated timestamp to current UTC

Why

plot_type audit: subplot and multi-panel are not in the canonical plot_type vocabulary (per spec-tags-generator.md lines 28-45). The multi-panel aspect is already captured by mosaic (the layout style) + multi in features.

features audit: dashboard describes where the layout is used (an application), not a special property of the visualization itself. It's one of several applications mentioned in the spec, not the core feature.

Hard guarantees from the prompt

• id, issue, created unchanged
• No semantic changes (data shape, plot type, requirements identical)
• updated bumped to current UTC

Auto-merge enabled — will merge once required checks pass. The skip-gate in daily-regen prevents stacking polish PRs for this spec.