Add libFuzzer harnesses for parser, loader, scanner and emitter APIs

[VAHPEM]

This PR adds experimental libFuzzer harnesses for several libyaml APIs.

Included targets:

• yaml_parser_scan()
• yaml_parser_parse()
• yaml_parser_load()
• yaml_emitter_emit()
• nested emitter harness
• parse → emit round-trip harness

Local fuzzing experiments show high parser-side coverage:

parser.c ≈ 92.9% line coverage
reader.c ≈ 94.8% line coverage
scanner.c ≈ 94.0% line coverage

The round-trip harness significantly improves emitter exploration:

emitter.c ≈ 85.3% line coverage

These harnesses may also serve as a starting point for future fuzzing
integration (for example OSS-Fuzz or similar continuous fuzzing setups).

A README in the fuzz/ directory documents the harnesses and the coverage
observations.

An example round-trip edge case discovered during early experimentation is
included for reference.