Add detailed bug fix plan for shuffle skew executor implementation

- Identified 5 bugs: 2 critical, 2 medium, 1 low priority
- Bug #1: Resource profile index mismatch (line 408)
- Bug apache#2: Tasks array indexing wrong (lines 420, 426)
- Bug apache#3: Incorrect average calculation (line 1341)
- Bug apache#4: Missing minimum threshold check (line 1316)
- Bug apache#5: Unclear counter semantics (line 121)

Plan includes:
- Detailed root cause analysis with concrete examples
- Step-by-step implementation instructions
- Index mapping approach (Option B) for Bug apache#2
- 6-commit strategy with tests passing between each
- Comprehensive test coverage additions

Author: warrenzhu25