Capture production-like RSC benchmark timing

[justin808]

Summary

Adds the next stacked benchmark/docs pass on top of #10.

• records /rsc_payload/ resource detail in benchmark JSON when the browser exposes a separate payload resource
• promotes RSC payload timing/transfer fields into the alternating comparison output
• documents the first production-like local benchmark using compiled Shakapacker/Rspack assets, compiled RSC demo bundles, no Shakapacker dev server, and a dedicated React on Rails Pro Node renderer
• updates README/status/performance handoff/upstream draft with the new result and caveats

Production-like benchmark result

production-like-alternating-8-reindexed, 8 alternating cycles, one warmup request per measured run, matching Chrome/ChromeDriver 147:

• median navigation: Inertia 775.40ms, RSC 607.15ms, -21.7%
• median LCP: Inertia 794.00ms, RSC 634.00ms, -20.2%
• median responseEnd: Inertia 644.80ms, RSC 588.80ms, -8.7%
• median action_total: Inertia 346.87ms, RSC 339.20ms, -2.2%
• JS requests: Inertia 6, RSC 1, -83.3%
• caution: p95 responseEnd still favors Inertia, 730.62ms vs 768.25ms, +5.2%

The first long run wrote 14/16 samples and then hit a Selenium Net::ReadTimeout while loading the RSC route. The final summary was completed with --reuse-existing, which reused the completed per-run JSON files and measured the two missing samples.

Caveat

The current RSC route streams its payload inline, so it does not expose a browser resource named /rsc_payload/ on initial load. The new resource timing fields are therefore empty for this implementation until we expose a separate payload resource or add renderer-internal timing.

Validation

• ruby -c scripts/perf/measure_dashboard.rb
• ruby -c scripts/perf/compare_dashboard_routes.rb
• payload-summary Ruby smoke test for rscPayload.resources and resource-level serverTiming
• bundle exec rubocop scripts/perf/measure_dashboard.rb scripts/perf/compare_dashboard_routes.rb
• git diff --check
• RAILS_ENV=production NODE_ENV=production bin/shakapacker
• RAILS_ENV=production NODE_ENV=production npm run build:rsc-demo
• DISABLE_SPRING=1 OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES bin/rails runner 'DevTools.delete_all_indices_and_reindex_all'
• ruby scripts/perf/compare_dashboard_routes.rb --base-url https://gumroad.dev --measure-base-url https://gumroad.dev --path /dashboard/inertia_demo --path /dashboard/rsc_demo --label production-like-alternating-8-reindexed --cycles 8 --server-warmup-requests 1 --require-driver-match --reuse-existing

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Note

Low Risk
Low risk: changes are limited to benchmark harness scripts and documentation, with no impact on app runtime behavior beyond how performance results are measured and reported.

Overview
Adds production-like benchmarking guidance and results across README and performance docs, including a new compiled-assets + dedicated renderer run and explicit caveats about tail latency (p95 responseEnd) and inline-streamed RSC payloads.

Enhances the perf harness to capture /rsc_payload/ resource timing/transfer details (including resource-level serverTiming) and promotes these fields into the alternating comparison output (compare_dashboard_routes.rb), alongside small Ruby refactors for cleaner collection/summarization.

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• Commit unit tests in branch jg-codex/production-like-rsc-profiling

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