Add cache_bust: option for automatic cache invalidation

[eclectic-coding]

Summary

• Adds cache_bust: callable option to memoize — a version token derived from instance state is folded into the cache key; when the token changes the old key produces no match and the method recomputes automatically
• Accepts a zero-argument Proc/lambda (invoked via instance_exec, with full access to self and instance variables) or a Symbol naming an instance method
• Returns any comparable value as the token: Time, Integer, String, Array, etc.
• Incompatible with key:; composes with namespace:, ttl:, if:, unless:, and shared_cache:

Usage

# ActiveRecord — cache busts automatically on every save
memoize :summary, cache_bust: -> { @order.updated_at }

# Symbol form
memoize :data, cache_bust: :cache_version

# Compound token
memoize :report, cache_bust: -> { [version, tenant_id] }

How it works

The token is incorporated as a 2-element custom key [method, [frozen_args, frozen_kwargs, token]]. Old token entries accumulate as stale until they expire via ttl: or are cleared by reset_memo. No new cache path code — reuses the existing custom-key infrastructure entirely.

Test plan

• bundle exec rake — 763 examples, 0 failures, lint clean
• spec/cache_bust_spec.rb — 18 new examples covering basic invalidation, method arguments, Symbol form, compound tokens, introspection transparency, TTL/namespace/shared_cache composition, and validation

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