ci(loader): pin vcpkg baseline so the Qt cache stops missing

The vcpkg install was cloned at a floating HEAD while native/vcpkg.json
carried no builtin-baseline, so every run resolved the qtbase port afresh.
Its package ABI drifted whenever upstream vcpkg moved, the restored
actions/cache entry no longer matched, and qtbase was rebuilt from source
(~31 min) on every push. Because the cache key is static (hash of
vcpkg.json, unchanged since 05-23) the rebuilt tree was never saved back
either, so the miss repeated forever.

Pin both sides to vcpkg tag 2026.05.25
(d015e31e90838a4c9dfa3eed45979bc70d9357fc, qtbase 6.11.0):
  - native/vcpkg.json: add "builtin-baseline"
  - workflow: clone --branch 2026.05.25

Adding the baseline also changes the vcpkg.json hash, so this run gets a
fresh cache key, cold-builds Qt once, and saves it; subsequent runs match
the now-deterministic ABI and skip the qtbase build entirely.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/build-loader.yml

@@ -88,11 +88,19 @@ jobs:
       - name: Clone & bootstrap vcpkg
         shell: pwsh
         run: |
+          # Pin vcpkg to the SAME commit as native/vcpkg.json's
+          # "builtin-baseline". A floating HEAD clone made the qtbase port
+          # (and thus its package ABI) drift every time upstream vcpkg moved,
+          # so the restored Qt cache never matched and qtbase was rebuilt
+          # from source (~31 min) on every run. Tag 2026.05.25 ==
+          # d015e31e90838a4c9dfa3eed45979bc70d9357fc and ships qtbase 6.11.0.
+          # Keep this tag and the builtin-baseline in vcpkg.json in lockstep.
+          $vcpkgTag = '2026.05.25'
           $vcpkg = Join-Path $env:GITHUB_WORKSPACE 'vcpkg'
           if (Test-Path $vcpkg) {
               Write-Host "Reusing existing $vcpkg"
           } else {
-              git clone --depth=1 https://github.com/microsoft/vcpkg.git $vcpkg
+              git clone --depth=1 --branch $vcpkgTag https://github.com/microsoft/vcpkg.git $vcpkg
           }
           & "$vcpkg\bootstrap-vcpkg.bat" -disableMetrics
           & "$vcpkg\vcpkg.exe" version