Make Build Insights primary with a delivery-safe fallback

## Business value

Build Insights can become the primary experience without putting .NET delivery at risk if the transition is evidence-based and Build Analysis remains an immediate fallback.

## Expected outcome

Build Insights becomes primary after the comparison process reports no differences for five consecutive business days. The Phase 1 feature switch remains available to restore Build Analysis when resolving an unexpected Build Insights issue could risk .NET delivery.

## Acceptance criteria

- The comparison process from [#6645](https://github.com/dotnet/arcade-services/issues/6645) reports no differences for five consecutive business days.
- The comparison evidence and switchover decision are recorded.
- The feature switch from [#6646](https://github.com/dotnet/arcade-services/issues/6646) is used to make Build Insights the primary Known Issue content producer.
- Build Analysis remains ready as a tested fallback.
- An unexpected Build Insights issue triggers fallback to Build Analysis when continued impact or a rushed resolution could risk .NET delivery.
- The reason, owner, and time of every fallback decision are recorded and communicated.
- Build Insights becomes primary again only after the issue is resolved and confidence is restored.

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🤖 _Drafted by an AI agent on behalf of @michalpavelka._

Start by reading the comparison process in #6645 and the Phase 1 feature switch in #6646, then identify how Build Insights and Build Analysis produce Known Issue content. Done means five consecutive business days without differences are recorded, the switchover is communicated, Build Analysis is tested as a fallback, and fallback decisions include their reason, owner, and time.

Written by the indexing model from the issue text.