Add renderer transport probe (#3784)

## Summary
- add benchmark-only probe comparing Fastify h2c TCP, native
`node:http2` h2c TCP, and native `node:http2` over Unix socket
- document the probe under `react_on_rails_pro/scripts/load`
- keep production renderer defaults/config untouched

Refs #3582
Refs #3583

## Codex Decision Log
- Kept this as Phase 0 evidence only; no Fastify replacement or UDS
production transport is selected here.
- Added cleanup/body-limit safeguards so custom payload and skip-UDS
runs do not produce misleading results or remove caller-owned files.
- Local benchmark evidence is macOS/same-host microprobe only;
Linux/topology validation against the real renderer harness remains
required before product decisions.

## Local benchmark note
Full local probe at `/tmp/ror-transport-probe-full` (3000 requests, 300
warmup, zero failures):
- small_unary p95: Fastify TCP 0.806ms, native TCP 1.090ms, native UDS
0.392ms
- stream_16kb p95: Fastify TCP 0.762ms, native TCP 0.729ms, native UDS
0.444ms

## Validation
- `bundle exec rspec spec/load/transport_probe_spec.rb`
- `bundle exec ruby scripts/load/transport_probe.rb --requests 1
--warmup 0 --body-bytes 1048577 --stream-bytes 1024 --scenarios
fastify_tcp --output-dir /tmp/ror-transport-probe-large-body-current`
- `bundle exec ruby -c scripts/load/lib/transport_probe.rb`
- `node --check scripts/load/transport_probe_server.mjs`
- `bundle exec rubocop --ignore-parent-exclusion
scripts/load/lib/transport_probe.rb scripts/load/transport_probe.rb
spec/load/transport_probe_spec.rb`
- `pnpm exec prettier --check react_on_rails_pro/scripts/load/README.md
react_on_rails_pro/scripts/load/transport_probe_server.mjs`
- `pnpm exec eslint
react_on_rails_pro/scripts/load/transport_probe_server.mjs`
- worker also ran `bundle exec rspec spec/load/`
- `codex review --base origin/main`

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Benchmark-only tooling under `scripts/load` with no production
transport or renderer behavior changes; risk is limited to monorepo
workspace/lockfile updates and local probe execution.
> 
> **Overview**
> Adds a **benchmark-only** renderer transport probe under
`react_on_rails_pro/scripts/load` to gather Phase 0 evidence for issues
#3582/#3583. It does **not** change production renderer defaults, Rails
HTTP clients, or runtime transport configuration.
> 
> A Ruby CLI (`transport_probe.rb` + `lib/transport_probe.rb`) spawns a
small Node server (`transport_probe_server.mjs`), drives HTTP/2 probe
traffic via `Async::HTTP`, and compares **Fastify h2c TCP**, **native
`node:http2` h2c TCP**, and **native HTTP/2 over Unix domain sockets**
on `small_unary` and `stream_response` workloads. Results include
latency percentiles, RPS, failure counts, baseline selection
(`fastify_tcp` preferred, else `native_tcp`), and per-scenario deltas;
output goes to `transport_probe_summary.json` under
`tmp/load-tests/transport-probe/`.
> 
> Monorepo wiring adds `react_on_rails_pro` and
`react_on_rails_pro/scripts/load` to pnpm workspaces with a private
`fastify` devDependency. The load README documents usage, `--skip-uds`,
body-limit semantics, and benchmark caveats. A large RSpec suite covers
config parsing, summary math, Node readiness validation, and Node server
edge cases (body limits, socket path safety, aborted streams).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
5e927ff. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

## Release Notes

* **New Features**
* Added HTTP/2 transport probe tool for benchmarking request performance
across Fastify, native HTTP/2, and Unix domain socket transports with
configurable request counts and payload sizes.

* **Documentation**
* Added transport probe documentation with setup instructions,
command-line options, and output file location details.

* **Tests**
* Added comprehensive test suite for transport probe functionality and
edge cases.

* **Chores**
* Updated monorepo workspace configuration to include react_on_rails_pro
package.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

## Agent Merge Confidence

Mode: accelerated-rc
Score: 9/10
Auto-merge recommendation: yes
Affected areas: Pro benchmark tooling, Node renderer transport research,
monorepo workspace metadata
CI detector: `script/ci-changes-detector origin/main` ->
JavaScript/TypeScript plus uncategorized changes; full suite and
benchmarks recommended
Validation run:
- `cd react_on_rails_pro && bundle exec rspec
spec/load/transport_probe_spec.rb` -> 31 examples, 0 failures
- `cd react_on_rails_pro && bundle exec rubocop
scripts/load/transport_probe.rb scripts/load/lib/transport_probe.rb
spec/load/transport_probe_spec.rb` -> no offenses
- `pnpm exec eslint
react_on_rails_pro/scripts/load/transport_probe_server.mjs` -> pass
- `pnpm exec prettier --check
react_on_rails_pro/scripts/load/transport_probe_server.mjs` -> pass
- `script/check-pro-license-headers --self-test &&
script/check-pro-license-headers --check` on a temporary merge with
`origin/main` -> pass
- `script/ci-changes-detector origin/main` -> full suite + benchmarks
recommended
Review/check gate:
- Claude review: complete for
`5e927ff82805e8cfb256c28261b7c07629282f6a`, no unresolved review threads
- Cursor Bugbot: complete for `5e927ff82805e8cfb256c28261b7c07629282f6a`
- GitHub checks: complete for
`5e927ff82805e8cfb256c28261b7c07629282f6a`; skipped benchmark
confirmation/report rows, docs-format-check, and legacy Claude Code job
are expected non-running paths
Known residual risk: low; this is benchmark-only tooling and does not
change production renderer transport defaults
Finalized by: Claude Code Review check `claude-review` SUCCESS for
`5e927ff82805e8cfb256c28261b7c07629282f6a` (GitHub Actions job
80367689384)

Author: justin808

package.json

@@ -10,6 +10,8 @@
     "packages/react-on-rails-pro-node-renderer",
     "packages/create-react-on-rails-app",
     "react_on_rails/spec/dummy",
+    "react_on_rails_pro",
+    "react_on_rails_pro/scripts/load",
     "react_on_rails_pro/spec/dummy",
     "react_on_rails_pro/spec/execjs-compatible-dummy"
   ],

pnpm-lock.yaml

@@ -1,5 +1,7 @@
 packages:
   - 'packages/*'
   - 'react_on_rails/spec/dummy'
+  - 'react_on_rails_pro'
+  - 'react_on_rails_pro/scripts/load'
   - 'react_on_rails_pro/spec/dummy'
   - 'react_on_rails_pro/spec/execjs-compatible-dummy'

pnpm-workspace.yaml

@@ -54,6 +54,55 @@ requests before measured requests begin. All workers must finish warmup before m
 `--start-gate-timeout` controls how long to wait for them (default: 30 seconds). If bundle upload is
 slow on a cold renderer, adjust `--upload-timeout` (default: 10 seconds).
 
+### Transport Probe For Fastify/Native HTTP/2/UDS
+
+Issues #3582 and #3583 need benchmark evidence before any production renderer
+transport change. The benchmark-only transport probe starts a tiny local Node
+server with equivalent probe endpoints over:
+
+- Fastify h2c over TCP loopback.
+- Native `node:http2` h2c over TCP loopback.
+- Native `node:http2` over a Unix domain socket.
+
+It does not change the production renderer, the Rails HTTP client, or the
+default h2c-over-TCP transport.
+
+Run from `react_on_rails_pro/` after workspace Node dependencies are installed:
+
+```bash
+bundle exec ruby scripts/load/transport_probe.rb --requests 3000 --warmup 300
+```
+
+Use Node >= 18, matching the Pro package requirement. The native stream probe
+uses Node's `Readable#iterator` API.
+
+`--body-bytes` is both the request payload size sent by each probe request and
+the server-side body limit. For example, `--body-bytes 1048576` sends a 1 MB
+request body and rejects requests larger than 1 MB. Normal benchmark requests
+send exactly the configured limit, so rejection testing requires a separate
+oversized request rather than a standard probe run.
+
+When comparing `stream_response` deltas, remember that Fastify enforces
+`bodyLimit` before the handler sees the request, while the native HTTP/2 stream
+handler reads the request body before writing the response. Treat that difference
+as part of the benchmark interpretation rather than production transport proof.
+
+For environments where Unix sockets are unavailable:
+
+```bash
+bundle exec ruby scripts/load/transport_probe.rb --skip-uds
+```
+
+The probe writes `transport_probe_summary.json` under
+`tmp/load-tests/transport-probe/<timestamp>/`. Treat this as Phase 0 evidence
+only: a production UDS transport still needs Linux/topology validation against
+the real renderer harness (`bin/renderer-harness`) before adding socket-path
+configuration or runtime transport plumbing.
+
+Summary JSON records the latency-delta baseline in the top-level `baseline`
+field. `fastify_tcp` is preferred when selected; if it is omitted, the probe
+falls back to `native_tcp` as the baseline.
+
 ### Tracking the node-renderer process
 
 To include the node-renderer RSS in `memory.csv`, pass its PID: