feat: enable metacopy=on to test metadata-only copy-up

Mount OverlayFS with metacopy=on option (kernel 4.2+, supported on ubuntu-22.04).
This prevents full file copy-up when files are opened with O_RDWR but not modified.

Expected behavior:
- ccache opens cache files with write access
- OverlayFS creates metadata-only entry in upper layer
- Full copy-up only happens if data is actually written
- Should dramatically reduce delta sizes from ~324 MB to ~KB

Re-enabled use-deltas for ccache to test this optimization.
Conan remains base-only (hash-based keys mean exact match most of the time).

If successful, deltas should be tiny for cache hit scenarios.

.github/actions/xahau-actions-cache-restore/action.yml

@@ -274,11 +274,12 @@ runs:
           exit 0
         fi
 
-        # Mount OverlayFS
+        # Mount OverlayFS with metacopy=on to prevent copy-up for files opened-but-not-written
+        # This dramatically reduces delta sizes when ccache opens files with O_RDWR but doesn't modify content
         echo ""
-        echo "Mounting OverlayFS..."
+        echo "Mounting OverlayFS with metacopy=on..."
         sudo mount -t overlay overlay \
-          -o lowerdir="${CACHE_WORKSPACE}/base",upperdir="${CACHE_WORKSPACE}/upper",workdir="${CACHE_WORKSPACE}/work" \
+          -o lowerdir="${CACHE_WORKSPACE}/base",upperdir="${CACHE_WORKSPACE}/upper",workdir="${CACHE_WORKSPACE}/work",metacopy=on \
           "${CACHE_WORKSPACE}/merged"
 
         # Verify mount

.github/actions/xahau-ga-build/action.yml

@@ -76,7 +76,7 @@ runs:
           ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ inputs.main_branch }}
           ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-
           ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-
-        use-deltas: 'false'  # Disabled: OverlayFS file-level deltas incompatible with ccache's byte-level LRU updates
+        use-deltas: 'true'  # Testing: metacopy=on should prevent copy-up for files opened-but-not-written
         aws-access-key-id: ${{ inputs.aws-access-key-id }}
         aws-secret-access-key: ${{ inputs.aws-secret-access-key }}
 
@@ -194,6 +194,6 @@ runs:
       with:
         path: ~/.ccache
         key: ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ steps.safe-branch.outputs.name }}
-        use-deltas: 'false'  # Disabled: OverlayFS file-level deltas incompatible with ccache's byte-level LRU updates
+        use-deltas: 'true'  # Testing: metacopy=on should prevent copy-up for files opened-but-not-written
         aws-access-key-id: ${{ inputs.aws-access-key-id }}
         aws-secret-access-key: ${{ inputs.aws-secret-access-key }}