Reuse memory data blocks in HexWidget to eliminate redundant IO reads

[andersendsa]

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Detailed description

• Modified MemoryData::fetch in src/widgets/HexWidget.h.
• Avoided redundant Core()->ioRead calls by reusing previously fetched QByteArray blocks.
• Takes advantage of Qt's implicit sharing for O(1) performance.
• Resolves the existing // FIXME: reuse data if possible comment.

Test plan (required)
1. Basic Functional Verification (UI/Scrolling):
Build Cutter with this branch and open any moderately sized binary.
Navigate to the Hexdump widget.
Scroll smoothly up and down using the mouse wheel, and also use the scrollbar to jump to different sections (both small contiguous steps and large jumps).
Expected Outcome: The memory values should display correctly without any visual corruption, missing chunks, or crashes. Scrolling should feel smooth and responsive.
2. View Resizing & Cache Edge Cases:
Resize the Hexdump widget window significantly (make it much taller and wider) to trigger variable block length fetches.
Expected Outcome: The widget should correctly fetch and stitch together the new blocks along with the reused blocks without rendering artifacts.
3. Developer/Performance Verification (Optional but recommended to prove the optimization):
Temporarily add qDebug() << "ioRead called"; inside the Core()->ioRead fallback loop in MemoryData::fetch (src/widgets/HexWidget.h).
Scroll one line down in the hex view.
Expected Outcome (Master branch): You will see multiple ioRead called prints equivalent to the entire visible screen's blocks.
Expected Outcome (This PR): You will see zero or only one ioRead called print, proving the partial cache overlap is successfully reusing the existing QByteArray blocks.

Closing issues