Summary

Adds Hash#fetch(key[, default]) method.

This is useful for reading configuration values safely. When a required key is missing and no default is given, fetch raises an error instead of returning nil silently. This helps catch mistakes early.

config = {timeout: 30, retries: 3}

config.fetch(:timeout)       #=> 30
config.fetch(:missing, 10)   #=> 10
config.fetch(:missing)       #=> raises error

Supports KeyError semantics (raises RuntimeError since KeyError is not available in mruby/c).

Changes

• Add c_hash_fetch function in src/c_hash.c
• Register fetch method in Hash class