[Core] generated_stylesheet_hrefs_json runs for OSS apps on every auto-loaded component render, but only the Pro client reads data-generated-stylesheet-hrefs

## Summary

`generated_stylesheet_hrefs_json` (added in #4047) runs on **every** `react_component` render that uses `auto_load_bundle`, for OSS and Pro alike — but the attribute it emits, `data-generated-stylesheet-hrefs`, is consumed only by the Pro client package (`packages/react-on-rails-pro/src/ClientSideRenderer.ts`, FOUC reveal gating). For OSS users this is pure hot-path waste: in development (Shakapacker `cache_manifest: false`) each call re-reads and JSON-parses `manifest.json` per component per request; in production it computes hrefs and emits a JSON attribute that nothing ever reads.

## Affected code

- `generated_stylesheet_hrefs_json`: https://github.com/shakacode/react_on_rails/blob/5264192ccf8897e4340380183d891feae908b94f/react_on_rails/lib/react_on_rails/pro_helper.rb#L50-L57
- Attribute emission on the component spec tag: https://github.com/shakacode/react_on_rails/blob/5264192ccf8897e4340380183d891feae908b94f/react_on_rails/lib/react_on_rails/pro_helper.rb#L24-L25
- Manifest lookup per call: https://github.com/shakacode/react_on_rails/blob/5264192ccf8897e4340380183d891feae908b94f/react_on_rails/lib/react_on_rails/helper.rb#L536-L550
- Runs for every render (ProHelper is included in the OSS helper; `generate_component_script` is called from `internal_react_component`): https://github.com/shakacode/react_on_rails/blob/5264192ccf8897e4340380183d891feae908b94f/react_on_rails/lib/react_on_rails/helper.rb#L21 and https://github.com/shakacode/react_on_rails/blob/5264192ccf8897e4340380183d891feae908b94f/react_on_rails/lib/react_on_rails/helper.rb#L802
- Sole consumer (Pro package only): https://github.com/shakacode/react_on_rails/blob/5264192ccf8897e4340380183d891feae908b94f/packages/react-on-rails-pro/src/ClientSideRenderer.ts#L49 and https://github.com/shakacode/react_on_rails/blob/5264192ccf8897e4340380183d891feae908b94f/packages/react-on-rails-pro/src/ClientSideRenderer.ts#L132-L136

## Introduced in

PR #4047 ("Fix Pro FOUC reveal gating"), merged 2026-06-15 (commit `af6525d8b`). Confirmed via `git log --oneline -S "generated_stylesheet_hrefs_json" 43ed0e138..origin/main` — single hit.

## Mechanism

1. Every `react_component` / `react_component_hash` call goes through `internal_react_component` → `generate_component_script(render_options)` (`ProHelper` is unconditionally included into `ReactOnRails::Helper`, so OSS apps run it too).
2. `generate_component_script` computes the attribute for every auto-loaded component:

```ruby
"data-generated-stylesheet-hrefs" =>
  generated_stylesheet_hrefs_json(render_options))
```

```ruby
def generated_stylesheet_hrefs_json(render_options)
  return unless render_options.auto_load_bundle

  pack_name = "generated/#{render_options.react_component_name}"
  sources = preload_sources_for_stylesheet_pack(pack_name)
  hrefs = unique_preload_sources_by_href(sources).map { |source| source.fetch(:href) }
  hrefs.to_json if hrefs.present?
end
```

3. `preload_sources_for_stylesheet_pack` → `preload_sources_for_pack` → `current_shakapacker_instance.manifest.lookup_pack_with_chunks(pack_name, type: :stylesheet)`. With Shakapacker's `cache_manifest: false` (the development default), each `lookup_pack_with_chunks` call refreshes the manifest — a `File.read` + `JSON.parse` of `manifest.json` — so this happens once per auto-loaded component per request, on top of the lookups the framework already does for pack tags.
4. The only reader of the resulting `data-generated-stylesheet-hrefs` attribute is `ClientSideRenderer.ts` in `packages/react-on-rails-pro` (`GENERATED_STYLESHEET_HREFS_ATTRIBUTE`, used by `generatedStylesheetMatchesComponent` for FOUC reveal gating). The OSS client package never reads it (verified by grep across `packages/`).

## Failure scenario

Not a correctness failure — a per-render performance/waste regression:

- OSS app in development with several auto-loaded components per page: each page render performs N extra full `manifest.json` read+parse cycles (manifest files in real apps are commonly hundreds of KB), inflating the dev feedback loop.
- OSS app in production: extra manifest hash lookups, href de-duplication, `to_json`, and dead HTML bytes (a JSON array of stylesheet URLs) on every component spec tag — payload that no client code consumes.

## Impact

- P2 performance/cleanliness: measurable dev-mode overhead for OSS apps with `auto_load_bundle`, plus dead-weight HTML in production responses. No behavior break.
- Pro apps pay the same dev-mode re-read cost per component even though a single per-request (or per pack name) memoization would suffice.

## Suggested fix

Two independent, compatible changes:

1. Early-return for OSS, since only the Pro client consumes the attribute:

```ruby
def generated_stylesheet_hrefs_json(render_options)
  return unless ReactOnRails::Utils.react_on_rails_pro?
  return unless render_options.auto_load_bundle
  ...
end
```

2. Memoize the href computation per pack name per request (mirroring other per-request caches in the helper):

```ruby
def generated_stylesheet_hrefs_cache
  @generated_stylesheet_hrefs_cache ||= {}
end

def generated_stylesheet_hrefs_json(render_options)
  return unless ReactOnRails::Utils.react_on_rails_pro?
  return unless render_options.auto_load_bundle

  pack_name = "generated/#{render_options.react_component_name}"
  generated_stylesheet_hrefs_cache.fetch(pack_name) do
    sources = preload_sources_for_stylesheet_pack(pack_name)
    hrefs = unique_preload_sources_by_href(sources).map { |source| source.fetch(:href) }
    generated_stylesheet_hrefs_cache[pack_name] = hrefs.present? ? hrefs.to_json : nil
  end
end
```

(Helper instances are per-request in Rails, so an ivar cache is request-scoped.)

## Test plan

- Spec (OSS mode, `react_on_rails_pro?` stubbed false): render an auto-loaded component; assert the spec tag has **no** `data-generated-stylesheet-hrefs` attribute and `preload_sources_for_stylesheet_pack` is not called.
- Spec (Pro mode): attribute still emitted with correct hrefs — existing #4047 coverage in `react_on_rails/spec/dummy/spec/helpers/react_on_rails_helper_spec.rb` should keep passing.
- Spec: two components sharing a generated pack name in one render only trigger one manifest lookup (memoization).
- Pro client test `packages/react-on-rails-pro/tests/ClientSideRenderer.test.ts` unaffected.
- Run: `cd react_on_rails && bundle exec rspec spec/dummy/spec/helpers/react_on_rails_helper_spec.rb`.

## Notes

- Verified against origin/main `5264192cc` by reading `pro_helper.rb`, `helper.rb:526-565`, and grepping all of `packages/` for the attribute — consumers are only the Pro `ClientSideRenderer.ts` and its test. Confidence: high. The dev-mode re-read claim rests on Shakapacker's documented `cache_manifest: false` behavior (manifest refreshed on each lookup), not re-measured here.
- A local fix session for this may already be queued (a task chip exists in the reporting workspace); this issue is the tracking artifact either way.
- Related finding (same review): "[Core] add_rsc_stream_observability_to_render_options writes through to the options hash held by RenderOptions" (same helper hot path).

_Found in a post-2026-06-12 code review of merged changes (review base 43ed0e138). Filed by Claude Code on behalf of Justin Gordon._


	def preload_sources_for_stylesheet_pack(pack_name)
	preload_sources_for_pack(pack_name, type: :stylesheet, required: false).map do \|source\|
	{ source:, source_type: :stylesheet }
	end
	end

	def preload_sources_for_pack(pack_name, type:, required:)
	manifest = current_shakapacker_instance.manifest
	sources = if required
	manifest.lookup_pack_with_chunks!(pack_name, type:)
	else
	manifest.lookup_pack_with_chunks(pack_name, type:)
	end
	Array(sources).compact
	end

	const generatedStylesheetHrefs = generatedStylesheetHrefsForComponent(componentSpec);
	// Generated stylesheet links are emitted into <head> before the component script runs.
	const stylesheetLinks = Array.from(
	document.querySelectorAll<HTMLLinkElement>('link[rel~="stylesheet"][href]'),
	).filter((link) => generatedStylesheetMatchesComponent(link, componentName, generatedStylesheetHrefs));

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[Core] generated_stylesheet_hrefs_json runs for OSS apps on every auto-loaded component render, but only the Pro client reads data-generated-stylesheet-hrefs #4341

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	def generated_stylesheet_hrefs_json(render_options)
	return unless render_options.auto_load_bundle

	pack_name = "generated/#{render_options.react_component_name}"
	sources = preload_sources_for_stylesheet_pack(pack_name)
	hrefs = unique_preload_sources_by_href(sources).map { \|source\| source.fetch(:href) }
	hrefs.to_json if hrefs.present?
	end

	"data-generated-stylesheet-hrefs" =>
	generated_stylesheet_hrefs_json(render_options))

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[Core] generated_stylesheet_hrefs_json runs for OSS apps on every auto-loaded component render, but only the Pro client reads data-generated-stylesheet-hrefs #4341

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