fix: false positive cycle detection for shared struct references

[Yanhu007]

Fixes #96

Problem

When two struct fields point to the same struct (shared reference, not a cycle), the second field is falsely flagged as (CYCLIC REFERENCE):

shared := &Inner{Value: 42}
o := Outer{A: shared, B: shared}
fmt.Sprintf("%# v", pretty.Formatter(o))
// B is falsely printed as: Inner{(CYCLIC REFERENCE)}

The visited map tracks all previously seen pointer addresses globally, but never removes them. So when a sibling field refers to the same struct, it appears as if we're revisiting an ancestor.

Fix

Delete the visit entry from visited after the struct is fully printed. This way only ancestors on the current recursion path are tracked, correctly distinguishing:

• Shared references (same pointer from sibling fields) → allowed
• Real cycles (pointer back to an ancestor) → detected

Verified both cases:

• Shared reference: no false positive ✅
• Real cycle (A→B→A): correctly detected ✅
• All existing tests pass ✅