docs(planning): JSON render-request body migration plan (#3280) (#3299)

Planning artifact for #3280 — **docs only, no code change**. Lives in
`internal/planning/` per repo convention (alongside
`MONOREPO_MERGER_PLAN.md`, `js-pro-package-separation-plan.md`, etc.).

Companion to experiment PR #3293. This is the production-quality
follow-up plan the issue asked for once we knew the numbers moved: it
records the change, the full benchmark results, the operational problems
hit while running the experiment, the deep compatibility/protocol
findings, and a phased production implementation plan.

> Supersedes the earlier (closed) #3298, which targeted
`upcoming-v16.3.0` and put the file under `react_on_rails_pro/`. This
one targets `main` and uses `internal/planning/`.

## What's in `internal/planning/json-render-body-migration-plan.md`

- **The change** — the 9-line `render_code` conditional and why no
renderer change is needed (Fastify default JSON parser;
`extractBodyArrayField` already handles both `key` and `key[]`; auth
reads `req.body.password` either way). Includes the runtime
instrumentation proof (Ruby↔Node byte-for-byte match on 3 payload
sizes).
- **Performance numbers** — sequential sweep, isolation, autocannon,
web-vitals, three-methodology robustness (sequential / paired-curl /
parallel-A/B), the #3217 (TCP_NODELAY) orthogonality re-verification,
and byte-equivalence. Headline: mega page **396 → 92 ms wall (−304),
112.8 → 9.8 ms GC (−103)**, ~2× the acceptance bar.
- **13 operational problems faced** — each with the workaround used.
- **Production plan** — compat matrix + protocol-version gating,
multipart preservation, streaming out-of-scope, tests, docs/changelog,
defensive audit, sequencing vs #3217.

## Links

| | |
|---|---|
| Issue | #3280 |
| Experiment PR (demo) | #3293 |
| Experiment commit | `352743ef1` |
| Demo-app harness commit |
[`b90977ef3`](AbanoubGhadban/rsc-benchmar@b90977e)
|
| Demo-app branch |
https://github.com/AbanoubGhadban/rsc-benchmar/tree/experiment/3280-json-render-body
|
| Orthogonal PR | #3217 |

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* **Documentation**
* Added planning document for a render request body format experiment:
includes experiment summary, benchmarking and correctness results,
operational findings, and a staged production implementation, testing,
and rollout plan.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: AbanoubGhadban