SSR (WEB_COMPUTE) build serves a stale/frozen build specification — ignores repo amplify.yml, console Build settings, UpdateApp/UpdateBranch, Reconnect, and branch recreate

### Environment
- Amplify Hosting, platform **WEB_COMPUTE** (Next.js - SSR), monorepo (`AMPLIFY_MONOREPO_APP_ROOT=apps/web`)
- Region: `us-east-1`
- App ID: `d13ckekjifghi7` (account details available on request)

### Summary
Every build runs a **stale 2-command `preBuild`**:

```
# Executing command: npm install -g pnpm@10.33.0
# Executing command: pnpm install --frozen-lockfile
```

regardless of the current build specification. The correct, updated spec (which adds a `preBuild` step that authenticates to a **private npm registry** before `pnpm install`) exists in **all** of the following, and the build ignores **every** one:

1. Repository root `amplify.yml` (which the console banner states overrides the console build settings).
2. `aws amplify update-app --build-spec ` — persisted; confirmed via `get-app`.
3. `aws amplify update-branch --build-spec `.
4. Console → **Build settings → Edit → Save** (the console reflects the new spec).
5. Console → App settings → **Reconnect repository**.
6. **Deleting and recreating the branch** — a brand-new branch (job counter reset to `1`) *still* ran the old 2 commands.

`get-app` returns the correct (updated) build spec, yet the build never runs it — the build's *effective* build specification appears **decoupled** from the app/branch/console buildSpec at the **app level**.

### Impact
Because the updated `preBuild` step never runs, `pnpm install` can't authenticate to the private registry (HTTP `401`) and the build fails. Production `main` builds have failed continuously since the private dependency was added.

### Reproduction
1. Update the build spec via any/all of the methods above.
2. Trigger a new build.
3. The build log shows only the old 2 commands, never the updated `preBuild` steps.

Verified across multiple commits, a fresh build after a console Save, and a brand-new branch created via delete + recreate.

### Expected
Builds use the **current** build specification — either the repo `amplify.yml` (per the documented override) or the saved console build settings.

### Ask
How do we force Amplify to re-read/refresh the *effective* build specification for an app + branch? Is there a known caching layer that survives `UpdateApp`, console Save, **Reconnect repository**, *and* branch delete/recreate? Short of recreating the entire app, what clears it?

Start with the repository root amplify.yml and compare it with the build specs returned by AWS CLI get-app, update-app, and update-branch. Inspect a fresh build log after the documented refresh actions. Done means the build consumes the current specification, including the private-registry authentication step, rather than the stale two-command preBuild.

Written by the indexing model from the issue text.