Fix undefined behavior in NaN encoding shift

[PJK]

Summary

• 0x7FC0 << 16 in cbor_encode_single() overflows signed int on systems with 32-bit int, which is undefined behavior per the C standard
• Cast to uint32_t before shifting to make it well-defined
• The cbor_encode_double() case ((uint64_t)0x7FF8 << 48) was already correctly cast

Test plan

• All 26 test binaries pass (existing NaN encoding tests cover this path)
• clang-format clean

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4b025b1) to head (11f313d).
⚠️ Report is 31 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #393   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         1661      1661           
=========================================
  Hits          1661      1661           

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