Add fixture inheritance with extends support

[ngan]

Summary

Adds fixture inheritance via extends:, enabling parent-child relationships that share setup data without duplication. Child fixtures can access parent exposed records through a parent helper and their cache files are self-contained (include all parent model records for replay).

Inheritance API

Named fixtures (spec/fixture_kit/child.rb):

FixtureKit.define(extends: "base") do
  project = Project.create!(name: "Child Project", owner: parent.owner)
  expose(project: project)
end

Inline fixtures (RSpec/Minitest):

fixture(extends: "base") do
  project = Project.create!(name: "Inline Project", owner: parent.owner)
  expose(project: project)
end

Multi-level chains work automatically — grandchild extends child extends base. Parent fixtures are generated before children. Only the child's explicitly exposed records are accessible; parent records are not auto-exposed.

Circular dependency detection

The Registry tracks a @resolving stack during fixture construction. If a name is encountered that's already on the stack, CircularFixtureInheritance is raised immediately at registration time (fail fast, no deferred errors).

Internal refactors

• Fixture#cache → Fixture#generate (the verb for triggering cache generation); Fixture#cache is now the attr_reader for the Cache object
• Registry#add(name_or_definition, scope) — name-first argument order
• Runner#register simplified to a passthrough to Registry
• RSpec/Minitest entrypoints now construct Definition objects themselves before calling register
• Cache uses MemoryData = Data.define(:records, :exposed) with model classes as hash keys instead of string names
• Repository#load_record uses { ModelClass => id } format with .keys.first / .values.first

Test plan

• All 112 unit + integration examples pass (bundle exec rspec)
• RSpec integration tests pass (FIXTURE_KIT_INTEGRATION_FRAMEWORK=rspec)
• Minitest integration tests pass (FIXTURE_KIT_INTEGRATION_FRAMEWORK=minitest)
• Both frameworks have identical assertion coverage (19 FKIT_ASSERT markers each)
• Inheritance chain: grandchild → child → base with record counts and association traversal
• Inline inheritance: anonymous fixture with extends: and parent helper
• Circular detection: CircularFixtureInheritance raised at registration time
• Parent records present in DB but not auto-exposed (NoMethodError verified)

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