Add auction telemetry addendum design spec

[jevansnyc]

What

Adds the 2026-07-30 auction telemetry addendum design spec as a reviewable document at docs/superpowers/specs/2026-07-30-auction-telemetry-addendum-dimensions-dashboard.md, alongside the parent design it amends (2026-06-22-auction-prebid-metrics-tinybird-grafana-design.md, shipped in #818).

This PR is docs only so engineering can comment line by line on the spec. No code, Tinybird, or Grafana changes are included here.

Why open it as a PR

The addendum specifies several workstreams: edge Rust signal propagation, fixed per-minute materialized views, hourly dimensional rollups and endpoints, health and reconciliation endpoints, and dashboard as code. Landing the spec first gives those implementation PRs a shared contract.

What the spec covers

• Rollup correctness: the three materialized-view null-filter fixes, conditional quantile handling, and explicit sample-count columns.
• New hourly dimensional rollups carrying country, device_class, browser_family, and os_family.
• Complete user_agent propagation from the edge, with browser and OS aggregation owned by Tinybird dimensional materialized views.
• Intentional raw User-Agent retention under the existing 30-day datasource TTL, including consent-denied observations.
• Health endpoints: real quarantine_counts plus event_reconciliation.
• Grafana dashboard-as-code panel definitions.

Implementation tracking

• Edge propagation and raw datasource schema: Pass user agent to auction telemetry #1103, which supersedes Emit browser_family device signal in auction telemetry #1039.
• Tinybird browser and OS derivation, rollups, endpoints, fixtures, and Grafana artifacts: follow-up implementation work under Auction telemetry addendum: rollup correctness, breakdown dimensions, and dashboard spec #979.

Tracks #979.

[aram356]

@jevansnyc to update the spec