ci: stop caching compiler-pinned CMakeCache on Windows (fixes runner-image-bump breakage)

Scheduled CI run 27544121035 (master, 2026-06-15) failed only on windows-x64 at
CMake PROJECT(): 'CMAKE_C_COMPILER .../MSVC/14.44.35207/.../cl.exe is not a full
path to an existing compiler tool'. Root cause: the 'Cache Windows CMake build
tree' step cached build/ci-windows/CMakeCache.txt + CMakeFiles/ + build.ninja,
which embed the absolute MSVC path, but the cache key hashed only source files —
not the runner image. Today's windows-2025 -> vs2026 image migration bumped MSVC
past 14.44.35207, so the restored CMakeCache pointed at a deleted compiler.

Fix: cache ONLY build/ci-windows/vcpkg_installed/ (the expensive deps) and let
CMake re-detect the compiler every run (cheap; objects still warm from the
separate ccache). Eliminates the pinned-compiler staleness class for all future
runner bumps. Key bumped to v3 to drop poisoned v2 entries. Supersedes the
deferred Phase 999.36 per-source-hash idea.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: okohlbacher

.github/workflows/ci.yml

@@ -257,28 +257,29 @@ jobs:
       # not from this cache. Windows-only; macOS+Linux configure in <30s already.
       # ci.yml itself is NOT in hashFiles() — editing this comment triggers a warm run
       # that exercises the cache-HIT path without invalidating the cmake-tree key.
-      - name: Cache Windows CMake build tree
+      - name: Cache Windows vcpkg deps
         if: matrix.os == 'windows-x64'
         uses: actions/cache@v5
         with:
+          # Cache ONLY vcpkg_installed/ — the expensive Boost/FFTW/etc. build.
+          # Do NOT cache CMakeCache.txt / CMakeFiles/ / build.ninja: those embed
+          # the absolute MSVC compiler path (e.g. .../MSVC/14.44.35207/.../cl.exe),
+          # while the key below hashes only source files, not the runner image.
+          # When GitHub bumps the windows runner image (the 2026-06-15 windows-2025
+          # → vs2026 migration bumped MSVC past 14.44.35207), the pinned compiler
+          # path disappears and a restored CMakeCache makes PROJECT() fail with
+          # "not a full path to an existing compiler tool" (see scheduled run
+          # 27544121035). Letting CMake re-detect the compiler each run — cheap
+          # once vcpkg_installed is warm; objects still come from the separate
+          # ccache — eliminates that whole failure class. (Supersedes the deferred
+          # Phase 999.36 per-source-hash invalidation idea, which this makes moot.)
+          # v3: drops the compiler-pinned paths from v2 and invalidates poisoned
+          # entries left by the runner-image bump.
           path: |
-            build/ci-windows/CMakeCache.txt
-            build/ci-windows/CMakeFiles/
-            build/ci-windows/build.ninja
             build/ci-windows/vcpkg_installed/
-          # v2 suffix forces cache invalidation after the cb392cc4 BALL_EXPORT
-          # masking bug (2026-05-17): the previous key didn't include source-
-          # file hashes, so source-only defects in DLL-export contracts could
-          # be masked by a warm cached CMakeFiles/. Bumping the version suffix
-          # forces a cold rebuild that catches the BALL_EXPORT restoration in
-          # 9042af07 and any similar future regression. Follow-up Phase 999.36
-          # is filed to add a per-source-hash invalidation arm.
-          # Phase 999.48 §8.7 step (b) — the prior ui_v2 suffix on this
-          # key (preventing OFF tree → ON build short-circuit) is no
-          # longer needed; the BALL_UI_V2 axis was removed with the flag.
-          key: ci-windows-cmake-tree-v2-${{ hashFiles('CMakeLists.txt', 'cmake/**', 'vcpkg.json', 'CMakePresets.json') }}
+          key: ci-windows-vcpkg-v3-${{ hashFiles('vcpkg.json', 'CMakePresets.json') }}
           restore-keys: |
-            ci-windows-cmake-tree-v2-
+            ci-windows-vcpkg-v3-
 
       # ======================================================================
       # macOS — Homebrew deps + the verbatim BUILD-macos.md flow.