fix: Use GetStableHashCode16 instead of truncation

[MrGadget1024]

GetStableHashCode16 uses XOR folding instead of truncation to reduce hash collisions, but we weren't using it everywhere.

• Updated Weaver code in 3 places
• Updated 3 weaver-called method signatures to ushort and removed casting
• Changed delegate registration in RemoteCalls
• Updated the GetDelegate test

Technical Justification

XOR folding preserves information from both halves of the 32-bit hash:
• Truncation example: 0x12345678 & 0xABCD5678 → both become 0x5678 ❌ Collision!
• XOR folding example:
• 0x12345678 → 0x1234 ^ 0x5678 = 0x444C
• 0xABCD5678 → 0xABCD ^ 0x5678 = 0xF9B5 ✅ Different results
This method is described in the official FNV documentation as the recommended approach for hash size reduction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.56%. Comparing base (5537326) to head (b835daf).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4102   +/-   ##
=======================================
  Coverage   43.56%   43.56%           
=======================================
  Files         150      150           
  Lines       14626    14626           
=======================================
  Hits         6372     6372           
  Misses       8254     8254           

Flag	Coverage Δ
unittests	43.56% <100.00%> (ø)
unity-6000.3.2f1	?
unity-6000.4.0f1	43.56% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
Assets/Mirror/Core/NetworkBehaviour.cs	85.71% <ø> (ø)
Assets/Mirror/Core/RemoteCalls.cs	90.24% <100.00%> (ø)

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