fix: prevent stack overflow on circular key references during value expansion - #412

When resolving variable references (%(name)s), circular references between keys (such as a = %(b)s and b = %(a)s, or self-references in the default section) result in infinite mutual recursive expansion during Key.String() evaluation, causing a fatal goroutine stack overflow crash.

Root Cause

Key.transformValue recursively called nk.String() to resolve nested variable substitutions without tracking visited keys or bounding the recursion depth across calls. In the presence of a reference cycle, this led to unbounded recursive calls.

Changes

1. Track visited *Key pointers along the expansion chain using hasVisitedKey and transformValueWithVisited.
2. Terminate expansion cleanly when a circular key reference or recursion depth exceeding depthValues (99) is encountered.
3. Stop repeating substitution when the resolved value contains the variable token itself, avoiding redundant loops on unresolvable references.
4. Add regression tests in key_test.go covering two-key mutual circular references, default section self-references, and multi-key cycles.

Fixes #409