Enable caching support for all components by default

[joelhawksley]

What are you trying to accomplish?

Enable caching support for all ViewComponents by default. Every component is integrated into Rails' template digest tree and can declare cache_on without including a module. ViewComponent::ExperimentallyCacheable is renamed to ViewComponent::Cacheable. The old name is removed without an alias, so existing include ViewComponent::ExperimentallyCacheable lines must be removed. Caching is no longer marked experimental in the guide or API docs. No module include or application-level opt-in is required for source dependency tracking, .cache_digest, digest-aware cache blocks inside component templates, or self-caching via cache_on.

This consolidates and supersedes #2711 and #2713, building on Erik Axel Nielsen's fixes for silently incomplete cache digests. The scope was subsequently expanded to make digest tracking automatic for every component, eliminating the application-wide activation side effect of including ExperimentallyCacheable on an unrelated component.

What approach did you choose and why?

• Install Action View dependency tracking and the synthetic resolver during engine initialization, without checking whether any component opted in.
• Register component subclasses from ViewComponent::Base and share registration for literal renders and explicit # Template Dependency: declarations. Store actual class names and virtual paths rather than reconstructing names with camelize, preserving acronym names and custom paths without retaining stale class objects across reloads.
• Move .cache_digest and component-template fragment digest integration to ViewComponent::Base. Direct digest computation also registers the component, including any overridden virtual path.
• Prepend Cacheable into ViewComponent::Base, so cache_on is available on every component. Explicit include ViewComponent::Cacheable is a harmless no-op (ActiveSupport::Concern skips modules already in the ancestor chain). Components still only cache their own output when they declare cache_on.
• Remove the now-meaningless __vc_cacheable? predicate.
• Preserve unexpected digest-error propagation from Raise errors while computing cache digests #2712. Dynamic renders still require explicit declarations, and partials continue using Rails' digest behavior.

This intentionally changes the default for all applications: existing Rails fragment cache blocks involving components now include component source dependencies, and every component responds to cache_on/cache_key. Components without fragment blocks or cache_on do not start caching output.

Anything you want to highlight for special attention from reviewers?

Regression coverage includes template and Ruby dependencies without caching opt-in, explicit declarations, transitive invalidation through a child with a custom virtual path to an acronym-named grandchild, actual fragment output after source changes, discovery starting from an empty registry, direct digest computation without prior registration, and autoload error propagation.

Component-hosted fragment fixtures no longer include the caching module, so existing exact-key, inherited-template, invalidation, and partial fall-through tests now exercise automatic behavior. The view fragment fixture renders only components without caching opt-in.

The sandbox's Zeitwerk inflector maps http_untracked_component to HTTPUntrackedComponent without changing Active Support's global inflections, exercising names that cannot be recovered with path.camelize.

Validation:

• Full default suite: 662 Minitests, 5 engine compatibility tests, and 8 RSpec examples, all passing.
• Rails 7.1 caching suites: 99 tests / 218 assertions, all passing.
• Full StandardRB, ERB lint, caching guide prose lint, and git diff --check pass.
• Fresh minimal Rails application with no caching opt-in installs both tracking hooks and computes a plain component's digest.
• Regenerated public API documentation for .cache_digest.

Erik is credited as a co-author for the original proposals and regression coverage.