Add Tag Breakdown columns to the Issues (Errors & Warnings) list view

### Problem Statement

When triaging issues in the Errors & Warnings (Issues) stream, there's no way to see tag distribution breakdowns inline without clicking into each individual issue.

For example, if I want to quickly scan my issue list and understand what percentage of events for each issue come from a specific tag value (e.g., platform:android vs platform:ios, or environment:production vs environment:staging), I currently have to click into each issue → Tags tab → find the tag → read the breakdown. This is extremely slow for triage at scale.

The tag breakdown data already exists — it's visible on the individual issue detail page as the "group by" tag breakdown showing percentages. But surfacing it as a column in the issue list view would make triage significantly faster.

**Workaround considered:** Discover/Explore can group events by tags, but it operates at the event level, not the issue level. It doesn't give you the same "here are my top issues, and for each one, here's how the tag breaks down as a percentage" view. It's a fundamentally different query shape.

### Solution Brainstorm

Allow users to add "Tag Breakdown" as a configurable column in the Issues list view (Errors & Warnings). When configured, the column would display a compact distribution visualization (e.g., a mini progress bar or percentage split) for a user-selected tag key, similar to how tag distributions are shown on the issue detail page's Tags tab.
This could be implemented as:

A new column type in the issue stream column picker (e.g., "Tag Breakdown: ")
A compact inline visualization showing the top 2-3 values with percentages (e.g., android 62% · iOS 35% · other 3%)
Clicking the breakdown could deep-link to the full tag detail for that issue

This would be especially valuable for game development and mobile teams that ship across multiple platforms and need to quickly assess whether an issue is platform-specific, environment-specific, or affecting a specific user segment — all without leaving the triage view.

### Product Area

Issues

Start with the Errors & Warnings Issues list view and the individual issue detail page’s Tags tab, where the existing group-by tag percentages are shown. Trace how the list’s configurable columns are defined and how tag breakdown data is loaded. Done means the scope is agreed and the issue list can present a selected tag breakdown without opening each issue; no files or tests are named in the report.

Written by the indexing model from the issue text.