fix: improve data.pack scan reliability and path handling

[ibraved]

Problem

Scan Tree was missing many files from data.pack.

The likely causes were in scanning/parsing robustness:

1. Header fields were read using direct pointer casts for 32-bit integers, which is brittle.
2. False-positive header matches could be accepted, causing cursor jumps by container_len and skipping valid entries.
3. Path handling accepted malformed path strings and relied on std::filesystem::path for in-pack virtual paths, which can behave unexpectedly for non-filesystem path data.

What This PR Changes

File changed: DataPack.cpp

1. Added safe LE integer reads with read_u32_le(...) instead of direct pointer casts.
2. Added sanitize_pack_path(...):
   • trims trailing NUL
   • normalizes \ to /
   • strips leading/trailing /
3. Added path plausibility filtering with is_likely_pack_path(...) before accepting entries.
4. Kept path_len validation aligned to byte max (<= 255).
5. Reworked AddFileToTree(...) to avoid std::filesystem::path for virtual in-pack paths:
   • manual path splitting
   • stable folder creation
   • extension detection from filename

Why This Fixes It

• Reduces false header acceptance and prevents invalid large cursor jumps that skip real files.
• Normalizes and validates path values before tree insertion.
• Handles in-pack paths as virtual paths rather than OS filesystem paths, avoiding accidental path interpretation side effects.

Validation

• Configured and built project successfully in Release.
• Executable starts successfully in smoke test.
• Manual UI verification should be done with the same data.pack repro case to confirm all expected files appear in Scan Tree.

Scope

• Focused only on scanner/path/tree logic in DataPack.cpp.
• No unrelated behavioral changes intended.