perf: disable delta caching (use base-only mode)

OverlayFS operates at FILE LEVEL:
- File modified by 8 bytes → Copy entire 500 KB file to delta
- ccache updates LRU metadata (8 bytes) on every cache hit
- 667 cache hits = 667 files copied = 324 MB delta (nearly full cache)

What we need is BYTE LEVEL deltas:
- File modified by 8 bytes → Delta is 8 bytes
- 100% cache hit scenario: 10 KB delta (not 324 MB)

OverlayFS is the wrong abstraction for ccache. Binary diff tools (rsync,
xdelta3, borg) would work better, but require significant rework.

For now: Disable deltas, use base-only caching
- Base cache restore still provides massive speed boost (2-5 min vs 20-30)
- Simpler, more reliable
- Saves S3 bandwidth (no 324 MB delta uploads)

Future: Research binary diff strategies (see Gemini prompt in session notes)

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

@@ -76,6 +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
         aws-access-key-id: ${{ inputs.aws-access-key-id }}
         aws-secret-access-key: ${{ inputs.aws-secret-access-key }}
 
@@ -193,5 +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
         aws-access-key-id: ${{ inputs.aws-access-key-id }}
         aws-secret-access-key: ${{ inputs.aws-secret-access-key }}