Investigate prebuild create vs claim build times in dogfood

## Description

In our dogfood environment, the “Write Coder on Coder” template has prebuilds enabled for certain presets. We recently introduced new metrics for prebuild creation time (building into the pool) and prebuild claim time https://github.com/coder/coder/pull/19503, and added them to our observability stack.

These metrics show that claim durations sometimes exceed creation by about 1–2 seconds. Because prebuilds maintain a ready-to-use pool to reduce workspace start time, claims should be consistently faster than creations.

We already use a Terraform `lifecycle` block with `ignore_changes` as recommended in the [docs](https://coder.com/docs/admin/templates/extending-templates/prebuilt-workspaces#preventing-resource-replacement), yet this pattern suggests Terraform may still be replacing or re-provisioning resources during claim, or the claim path is performing extra work.

For reference, see the attached screenshot comparing average build times for a prebuild creation versus a claim on the Falkenstein preset in our dogfood instance.

## Investigation scope

Investigate the source of the additional claim-path latency relative to the create path.

まず observability stack における prebuild の作成と claim メトリクスから始め、リンク先のドキュメントで説明されている Falkenstein の dogfood データと、ignore_changes を含む Terraform の lifecycle ブロックを比較します。追加の 1～2 秒の発生源を特定するために create と claim のパスを追跡します。完了の条件は、追加の claim レイテンシーが説明され、Terraform による replacement または reprovisioning が発生しているかどうかが確認または否定されることです。

索引モデルが issue の本文から書いたものです。