Reduce per-render allocations by 11 (3 inline, 8 collection)

[joelhawksley]

Rendering a ViewComponent is on the hot path for a lot of apps, so shaving off allocations there compounds across a request. Profiling a single render_in with ObjectSpace.trace_object_allocations surfaced a handful of hotspots that had nothing to do with template logic and everything to do with per-call config lookups, kwarg splats, and per-item work inside the collection loop.

Approach

• Cache the instrumentation-enabled flag. ViewComponent::Instrumentation#render_in was walking Rails.application.config.view_component.instrumentation_enabled on every render, which allocates two hashes each time. A module-level Instrumentation.enabled accessor is now set once at boot (in the engine initializer) and toggled by the test helper, so the fast path is a single ivar read. The **kwargs splat on render_in also went away.
• Hoist collection per-item work. Collection#components was calling component.__vc_collection_parameter, __vc_counter_argument_present?, etc. inside the loop and building a fresh item_options hash from @options.dup per item via a helper. Those metadata lookups are now hoisted above the loop, item_options is duped once and mutated per iteration, and __vc_validate_collection_parameter! is skipped once the component is compiled.
• Memoize the empty-details Requested. RequestDetails#vc_requested_details was rebuilding an ActionView::TemplateDetails::Requested (and the intermediate tuple) on every render even though user_details is almost always the frozen EMPTY_DETAILS. That result is now memoized on the LookupContext instance.
• Trim splats and reuse the empty spacer. with_collection passes options positionally into Collection.new, Collection#initialize takes a positional options hash, and the no-spacer render returns a frozen "".html_safe constant instead of allocating a fresh string.

Results

Measured against the existing RenderingAllocationsTest on Rails 8.1 / Ruby 4.0:

• Inline: 35 -> 32 (-3)
• Collection: 61 -> 53 (-8)
• Total: -11

The same -3 / -8 delta is applied to every Rails/Ruby pair in the allocation table. Full suite (624 runs, 1822 assertions) is green.

Notes for reviewers

• The Instrumentation.enabled module flag needs the engine initializer and the test helper to keep it in sync with config.view_component.instrumentation_enabled. Both are updated here; anywhere else that flips the config directly would also need to flip the module flag, but there aren't any such call sites in the repo today.
• Collection#component_options is inlined into components since it's no longer meaningfully separable once the metadata is hoisted.
• No public API changes: with_collection(...) still accepts the same kwargs; only the internal Collection.new signature moved from **options to a positional options = {}.