Setup fuzzing for merge function

[mroczect]

Summary

Integrate fuzzing for the three-way merge function (merge_trees) in libvctrl_plumbing. The goal is to ensure that the merge algorithm never panics or produces corrupted output when given arbitrary tree structures.

Context

Merge logic can be complex and error-prone. Fuzzing with random trees and ancestor combinations can reveal hidden bugs.

Tasks

• Create a fuzz target that:
  • Constructs random tree objects (with random blobs and paths) using the memory object store.
  • Calls merge_trees with the random ancestor, ours, and theirs trees.
  • Asserts that the function returns either Ok(MergeResult) or Err(VctrlError) without panicking.
  • Optionally verifies that successful merge results can be read back and decoded.
• Add the fuzz target to the existing fuzzing setup from issue 5.1.
• Run a short fuzz session to catch immediate crashes.
• Document the fuzz target and how to extend it.

Acceptance Criteria

• Fuzz target for merge exists and runs without crashing in initial tests.
• No panic is observed during fuzzing.

Dependencies

• Fase 3 plumbing merge_trees (3.5.3).
• Fuzz infrastructure from issue 5.1.