### Background
Configure and validate event logging in TestKitchen for the Donor badge experiment so we can measure the feature's impact and evaluate success criteria.
This covers registering the experiment, confirming bucket assignment works end-to-end, and verifying that exposure events reach the logging pipeline correctly for all three variants.
Spec events would include (not exhaustive at this time): tapping the donor button, multiple taps per user.
Measurement plan: https://docs.google.com/document/d/177uzZs5WyUNYyQcdD6zGQZ9pWBs0hh5HTXR0B6kbHDY/edit?tab=t.0
Instrumentation spec: https://docs.google.com/spreadsheets/d/1NYyinpkV3dswcamBJjLx0AW8P3KzefQyhD85VITIp7U/edit?gid=1635536298#gid=1635536298
| A: Control | B: Plain message | C: Delightful experience
| {F80974413} | {F80034142} | {F80034220} {F80034219}
### Goal
Experiment is registered, bucketing is verified end-to-end, and exposure events are confirmed in the pipeline before any variant-specific UI ships.
### Requirements
- Register experiment in [[ https://test-kitchen.wikimedia.org/experiment/donor-delight-badge | TestKitchen with variant identifiers A, B, C ]] (correct weights) and initial traffic split and targeting rules (logged-out, mobile web, post-donation only: donor cookie present).
- Bucket assignment verified as stable and deterministic (persistent at the edge uniques level) across sessions.
- Exposure event schema
- Verify exposure events in the logging pipeline for all three variants using test donor sessions.
=== Technical information
[[ https://test-kitchen.wikimedia.org/experiment/donor-delight-badge | Experiment Variations (TestKitchen) ]]
- Control (`control`)
- Treatment B: Simple (`treatment-b-simple`)
- Treatment C: Delightful (`treatment-c-delightful`)
Useful JS for testing
```lang=js
mw.user.clientPrefs.set( 'minerva-badge', '1' )
mw.testKitchen.overrideExperimentGroup( 'donor-delight-badge', 'treatment…' );
mw.testKitchen.overrideExperimentGroup( 'donor-delight-badge', 'control' );
```
### Acceptance Criteria for done
[x] Experiment registered in TestKitchen with correct targeting and variant weights. https://test-kitchen.wikimedia.org/experiment/donor-delight-badge
[] Exposure events firing and confirmed in the logging pipeline for variants A, B, and C.
[] Bucket assignment verified as stable across sessions for the same test donor token.
[] Data Engineering sign-off on exposure event schema before moving to instrumentation sub-tasks.