feat(gcp): tabbed Google Cloud dashboard with project, region and workload filters - #1496

🟡 Confidence 5/10 · needs attention
The engine and template additions are pinned by new tests, but the unescaped $project filter and the uncapped metric-name input are still open at this head.
_{quality 80/100 · 2 warnings · tests partial · risk medium · 1/1 new units observable}

Tabbed Google Cloud dashboard template with Project / Region / Workload query variables over a new metric variable source, plus a gcp service-map platform. Engine work is tested; the two earlier warnings still stand, and the generated lab fixture was only partly read.

• gcpTemplate gains Project/Region/Workload query variables and one tab per service
• New metric variable source lists attribute values from named metrics of one metricType
• Service map classifies and legends a gcp platform
• buildTemplatePreview stacks the root canvas and each section's first tab

Still open from earlier reviews

• 🟠 Warning · F1 · Project filter substitutes the project id unescaped into SQL · packages/backend/src/dashboard-templates/infrastructure/gcp.ts:79
• 🟠 Warning · F2 · Metric names input has no 20-item cap the dashboard schema enforces · apps/web/src/components/dashboard-builder/config/variables-manager-dialog.tsx:553

_{e2ceedd · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}