Refactor Unit class for improved code quality and maintainability (#382)

Author: olbrich

.reek.yml

@@ -0,0 +1,34 @@
+# Reek configuration
+#
+# InstanceVariableAssumption is disabled for Unit class because it uses a mix of
+# eager initialization (in initialize_instance_variables) and lazy initialization
+# (using defined? checks) for its instance variables. This is an intentional design
+# pattern where some variables are initialized immediately and others are computed
+# on first access for performance reasons.
+#
+# See: https://github.com/troessner/reek/blob/master/docs/Instance-Variable-Assumption.md
+#
+# RepeatedConditional warnings are excluded for Unit class where the same condition
+# needs to be checked in different methods for different purposes:
+#
+# - base?: Checked in to_base (early return), update_base_scalar (base unit calculation),
+#   and unit_signature_vector (recursion avoidance). Each serves a distinct purpose.
+#
+# - other: Type checking in operator overloading methods using case statements.
+#   This is a standard Ruby pattern for implementing polymorphic operators.
+#
+# - prefix_value: Checked in to_base (numerator/denominator processing) and parse
+#   (string parsing). These are completely different contexts.
+#
+# - temperature?: Checked in multiple methods (to_base, temperature_scale, arithmetic
+#   operators, power/root) where each serves a different purpose (conversion, formatting,
+#   validation, etc.).
+
+detectors:
+  InstanceVariableAssumption:
+    exclude:
+      - RubyUnits::Unit
+
+  RepeatedConditional:
+    exclude:
+      - RubyUnits::Unit

lib/ruby_units/cache.rb

@@ -3,7 +3,7 @@
 module RubyUnits
   # Performance optimizations to avoid creating units unnecessarily
   class Cache
-    attr_accessor :data
+    attr_reader :data
 
     def initialize
       clear