refactor(FileDataBuffer): O(1) LRU cache, fix hash mismatch, deduplicate save logic

[lagcompensation]

• Replace QList-based LRU with std::list + QHash for O(1) chunk touch/evict
• Add touchChunkLocked / trimChunkCacheLocked implementations
• Unify computeCurrentHashLocked to use qHash in both modes (fixes isModified()
  returning wrong result after file-backed → memory transition)
• Extract reopenSource lambda in saveToFile to remove triplicated recovery blocks
• Skip O(n) data comparison in replaceData if sizes differ
• Fix stale m_chunkLru.clear() reference in openFile → use m_chunkLruList/Iter

[Altro-O]

FileDataBuffer.cpp:76-80, 422-434
The reported fix for hash mismatch is not complete. After replaceData(), the file-backed buffer is still forced to memory mode via promoteToMemoryModeLocked(), and computeCurrentHashLocked() still calculates the hash differently in the two modes: qHash(m_data, 0) for materialized and XOR from qHash(readLocked(...), offset) for file-backed. As a result, isModified() can still be true without actually changing the data, if the contents haven't changed, but only the storage mode has.

FileDataBuffer.h:4-10, 107-108
std::list and std::list<...>::iterator have appeared in the header, but #include has not been added. This can currently only be built accidentally due to transitive includes, but any TU that includes this header without them will get a compile error.

Please correct the errors and we will review them again.

[lagcompensation]

@Altro-O fixed