refactor: extract helper functions and simplify buildInodeTableToBuffer

Significantly improved code organization and maintainability:
- Extracted 11 helper functions from buildInodeTableToBuffer
- Reduced buildInodeTableToBuffer from 442 to ~115 lines (73% reduction)
- Eliminated code duplication between Pass 2 and Pass 4
- Added inodePosition and directoryChunk type definitions
- Enhanced documentation for Writer, writerInode, and Finalize
- Improved clarity with well-named extraction functions:
  - computeInodePositions, computeBlockPositions
  - buildDirectoryEntryData, buildDirectoryDataForAllInodes
  - serializeInodesToBuffer, writeCompressedMetadataBlock
  - simulateDirectoryIndices, rebuildDirectoryDataWithBlockPositions

All tests pass. No functional changes - purely refactoring.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: MagicalTux