feat: unified PR report combining bundle size and runtime perf

Replace separate pr-size-report.yaml and pr-perf-report.yaml with a
single pr-report.yaml aggregator that triggers on both CI workflow
completions. Produces one combined PR comment with bundle size and
performance sections, showing 'pending' placeholders for whichever
section hasn't completed yet.

Key changes:
- Add head-sha metadata to both CI workflows for stale-run detection
- Add concurrency to ci-size-data.yaml (matches ci-perf-report.yaml)
- Create unified-report.js that shells out to existing size-report.js
  and perf-report.ts, combining their output
- Clean up legacy separate comment markers on first run
- Use PR head SHA as concurrency key to prevent race conditions

Author: christian-byrne

.github/workflows/ci-perf-report.yaml

@@ -64,6 +64,7 @@ jobs:
           mkdir -p temp/perf-meta
           echo "${{ github.event.number }}" > temp/perf-meta/number.txt
           echo "${{ github.event.pull_request.base.ref }}" > temp/perf-meta/base.txt
+          echo "${{ github.event.pull_request.head.sha }}" > temp/perf-meta/head-sha.txt
 
       - name: Upload PR metadata
         if: github.event_name == 'pull_request'

.github/workflows/ci-size-data.yaml

@@ -8,6 +8,10 @@ on:
     branches:
       - main
 
+concurrency:
+  group: size-${{ github.ref }}
+  cancel-in-progress: true
+
 permissions:
   contents: read
 
@@ -28,11 +32,12 @@ jobs:
       - name: Collect size data
         run: node scripts/size-collect.js
 
-      - name: Save PR number & base branch
+      - name: Save PR metadata
         if: ${{ github.event_name == 'pull_request' }}
         run: |
           echo ${{ github.event.number }} > ./temp/size/number.txt
           echo ${{ github.base_ref }} > ./temp/size/base.txt
+          echo ${{ github.event.pull_request.head.sha }} > ./temp/size/head-sha.txt
 
       - name: Upload size data
         uses: actions/upload-artifact@v6