Setup fuzzing for decoder and encoder

[mroczect]

Summary

Integrate cargo-fuzz into the project to fuzz the binary codec (encoder and decoder) in libvctrl_core. Fuzzing helps discover panics, hangs, and memory safety issues in parsing and serialization logic.

Context

The binary codec is a critical component: it processes untrusted data (e.g., packfiles, object files). Fuzzing will exercise edge cases that unit tests may miss.

Tasks

• Add cargo-fuzz as a development dependency (or install via cargo install cargo-fuzz).
• Create a fuzz target in fuzz/fuzz_targets/ (or per crate) that:
  • Takes arbitrary bytes as input.
  • Attempts to decode them using BinaryDecoder.
  • If decoding succeeds, re-encode the result and compare (round-trip).
  • If decoding fails, ensure no panic occurs (only Err returned).
• Configure the fuzz target to run with cargo fuzz run <target>.
• Add a script or Makefile target to run fuzzing locally.
• Document how to run fuzzers in CONTRIBUTING.md or README.md.

Acceptance Criteria

• Fuzz target compiles and runs without crashing for at least 10 seconds (or a short smoke test).
• The fuzzer finds no panics in the initial run.
• Instructions for fuzzing are documented.

Dependencies

• Fase 2 core codec implementation (2.4).