Refactor reported design and code smell issues

[DevNexus09]

This PR applies behavior-preserving refactors for all 41 design issues.

Key changes include:

• Decomposed large worksheet/workbook orchestration methods into focused helpers
• Replaced duplicated validation serialization with a shared validation base
• Removed duplicated workbook/worksheet password hashing via LegacyProtectionHash
• Introduced typed writer cell value objects to reduce instanceof branching
• Extracted reader formula resolution from RowSpliterator
• Encapsulated reader format metadata with FormatCatalog
• Made XML parser factory usage explicit instead of static/global
• Added a small fastexcel-core module for shared cell-address conversion
• Reduced mutable state leaks and unnecessary coupling
• Split broad compatibility test setup into named helper methods

Verification

• mvn -pl fastexcel-writer test passed
• mvn -pl fastexcel-reader test passed
• mvn test passed for the full reactor

Final full test results:

• Fastexcel Core: SUCCESS
• Fastexcel Writer: 111 tests passed
• Fastexcel Reader: 76 tests passed
• Fastexcel End-to-End: 12 tests passed

Notes

No functional behavior changes are intended.

[coveralls]

coverage: 89.067% (+0.3%) from 88.791% — DevNexus09:issues/code-smell-fix into dhatim:master