Fix bugs and improve robustness in example code

[PJK]

Summary

• Fix negint conversion bug in cbor2cjson.c: unsigned arithmetic on cbor_get_int() (returns uint64_t) caused wraparound; use double arithmetic instead
• Fix negint encoding bug in cjson2cbor.c: valueint + 1 produced wrong value for the negint64 callback; use (uint64_t)(-1 - valueint) per CBOR encoding rules
• Fix infinite loop in streaming_parser.c: if cbor_stream_decode returns CBOR_DECODER_ERROR, .read is 0 causing an infinite loop; add error check with break
• Fix map key buffer in cbor2cjson.c: replace fixed 128-byte buffer with silent truncation with dynamic allocation based on actual key length
• Replace confusing cbor_move pattern in cbor2cjson.c with explicit get/use/decref for clarity
• Add fread/sscanf return value checks across all 6 example files
• Clean up comments in cjson2cbor.c: replace unprofessional/unclear comments

Test plan

• cmake --build with -DWITH_EXAMPLES=ON succeeds
• All 26 tests pass
• clang-format.sh passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (dfe2297) to head (3b6a86b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #407   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           20        20           
  Lines         1663      1663           
=========================================
  Hits          1663      1663           

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