Honor Shakapacker dev server overrides

[justin808]

Summary

• make the custom Webpack and Rspack config honor SHAKAPACKER_DEV_SERVER_* overrides like Ruby/Shakapacker
• document the clean-port local validation workflow after fixing the 3035 asset-proxy mismatch
• promote the corrected clean-port 8-cycle benchmark as the current headline local result

Validation

• node --input-type=module ... for default and overridden Shakapacker dev-server ports
• manual browser verification of /dashboard/inertia_demo and /dashboard/rsc_demo on SHAKAPACKER_DEV_SERVER_PORT=3036
• curl -k -I https://app.gumroad.dev/packs/js/webpack-runtime.js
• scripts/perf/measure_dashboard.rb single-run smoke with matching ChromeDriver 147
• scripts/perf/compare_dashboard_routes.rb corrected clean-port 4-cycle rerun with --reuse-existing
• scripts/perf/compare_dashboard_routes.rb corrected clean-port 8-cycle rerun

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Note

Low Risk
Low risk: small config-loading refactor for dev-server settings plus documentation/perf-note updates; production runtime behavior should be unchanged aside from reading YAML via a shared helper.

Overview
Adds a shared JS loader (config/shakapackerConfig.js) for shakapacker.yml that applies SHAKAPACKER_DEV_SERVER_* environment overrides (with basic type parsing) so both the custom Webpack and Rspack configs stay in sync with Ruby/Shakapacker.

Updates README and performance docs to document the clean-port workflow (e.g., using SHAKAPACKER_DEV_SERVER_PORT=3036) and to promote the corrected clean-port 8-cycle benchmark numbers as the current headline, including notes on the prior 3035 asset-proxy mismatch.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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