Fill the DAP Impact Adoption view with GLQL panels

What does this MR do and why?

• Adds eleven GLQL panels to the previously empty Adoption view of the DAP Impact dashboard (/explore/analytics_dashboards/dap_impact), laid out to match the approved design's three sections: Users, Engagement and Adoption tiers.
• Four panels use the intended query: "Total users", "Users by activity", "Users across tiers" and "Total sessions".
• Seven panels are placeholder stand-ins, titled with a "(WIP)" suffix and a tooltip noting they show placeholder data, because the real metric, dimension or display type does not exist yet: "New vs returning (WIP)", "Multi-activity users (WIP)", "Abandonment (WIP)", "Sessions by tier and capability (WIP)", "Share of users vs share of sessions (WIP)", "Session intensity by tier (WIP)" and "Group comparison (WIP)".
• The "Total users" and "Multi-activity users (WIP)" stats set showTrends: true, so they show the previous-period badge from the design, using the support added in !253653 (merged).
• The per-row "vs last month" badges on "Users by activity" and "Group comparison" are left out, because GLQL has no offset data series yet.
• Section headings and their one-line descriptions from the design are left out, because the dashboard framework has no header visualization yet.
• The two single stats in the Engagement section show absolute numbers, not percentages, per product decision.
• Every panel uses the dashboard date range placeholders, so the date range filter applies to all of them.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/628024 (Total users stat)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628025 (New vs returning users bar chart)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628026 (Users by activity)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628027 (Multi-activity users stat)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628029 (Abandonment stat)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628031 (Sessions by tier and capability heat map)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628033 (Users across tiers stat)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628034 (Total sessions stat)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/628035 (Session intensity by tier table)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/623323 (Share of users vs share of sessions)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/623357 (Group comparison table)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/605725 (header visualization, not yet built)
• https://gitlab.com/gitlab-org/gitlab/-/work_items/624329 (GLQL offset data series, not yet built)
• !253653 (merged) (previous-period trend badge on GLQL stats)

Screenshots or screen recordings

Screenshots will be added after local review.

How to set up and validate locally

1. Seed AI usage events with FILTER=ai_usage_stats PROJECT_ID=2 SEED_AI_USAGE_STATS=1 bundle exec rake db:seed_fu (project 2 is gitlab-org/gitlab-test; ClickHouse must be enabled for analytics).
2. Run gdk restart rails-web, because system dashboard YAML is read once per process and cached.
3. Visit /explore/analytics_dashboards/dap_impact, pick the gitlab-org group in the filter at the top, and open the Adoption tab.
4. Confirm eleven panels render, that "Total users" and "Multi-activity users (WIP)" show a trend badge, and that changing the date range changes the values.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.