Add Measured::Temperature unit type

[ntn]

Summary

Adds Measured::Temperature as the first functional unit type, demonstrating the proc-based conversion system from #188.

Includes three units:

• Celsius (C) - base unit
• Kelvin (K) - celsius + 273.15
• Fahrenheit (F) - celsius * 9/5 + 32

Opt-in only

Temperature is not included in the default require "measured". Users opt in explicitly:

# In Gemfile
gem 'measured', require: ['measured', 'measured/temperature']

# Or manually
require 'measured/temperature'

This keeps the default gem lightweight and avoids loading functional conversion infrastructure for users who only need static units (length, weight, volume).

Usage

require 'measured/temperature'

temp = Measured::Temperature.new(100, :C)
temp.convert_to(:F)  # => 212 F
temp.convert_to(:K)  # => 373.15 K

Measured::Temperature.new(0, :K).convert_to(:C)  # => -273.15 C

Test plan

• All temperature conversions (C↔K, C↔F, K↔F, identity, negatives, indirect paths)
• Arithmetic and comparison across units
• Existing unit tests (length, weight, volume) unaffected
• require 'measured' alone does not load Temperature