[AdamMagued]

Fixes #184

Problem

When creating a sublogger via With, the logger struct was shallow-copied by value. This decoupled the sublogger's level field from the parent logger. Calling SetLevel on the parent logger after sublogger creation did not update the sublogger's active log level.

Solution

• Track parent-child logger relationships by adding a parent reference and a hasCustomLevel flag to Logger.
• Update level checks (Log, GetLevel, and Enabled) to resolve the effective level by traversing the parent chain until reaching an explicitly configured level.
• Set hasCustomLevel upon calling SetLevel, allowing subloggers to override their inherited level without mutating the parent or other siblings.
• Added comprehensive unit tests validating level inheritance, explicit child overrides, multi-level hierarchies, and concurrency safety.