docs(planning): record #3293 closure + related-PR status table (#3280)

Add §6 documenting that the experiment codebase PR #3293 is closed
(superseded by this plan), with a disposition table for #3293/#3298/this
PR, and an explicit note that orthogonal #3217 is left untouched.

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

Author: AbanoubGhadban

internal/planning/json-render-body-migration-plan.md

@@ -12,7 +12,7 @@ implementation still needs. It is not itself the implementation.
 | Artifact                               | Location                                                                                                                                                                                  |
 | -------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | Issue                                  | https://github.com/shakacode/react_on_rails/issues/3280                                                                                                                                   |
-| Experiment PR (the "demo")             | https://github.com/shakacode/react_on_rails/pull/3293                                                                                                                                     |
+| Experiment PR (the "demo") — **closed** | [#3293](https://github.com/shakacode/react_on_rails/pull/3293) (closed; experiment fully captured in this doc — see note below)                                                            |
 | Experiment commit (react_on_rails)     | [`352743ef1`](https://github.com/shakacode/react_on_rails/commit/352743ef18b204bb34072b42aa8be32f05927bfa) — `[EXPERIMENT] Send render request body as JSON when no bundle attached`      |
 | Experiment branch                      | `experiment/3280-json-render-body` (base `upcoming-v16.3.0`)                                                                                                                              |
 | Demo-app harness commit (rsc-benchmar) | [`b90977ef3`](https://github.com/AbanoubGhadban/rsc-benchmar/commit/b90977ef33d72c33aacbce09b07d66db46b46155) — `demo(#3280): JSON render-body experiment harness + parallel A/B tooling` |
@@ -359,3 +359,19 @@ re-litigate #3217's numbers, only note that the JSON win was re-verified with
 Open a single production PR implementing §4.1–§4.6, gated on a protocol
 version bump (§4.1) so old renderers keep getting form-encoded bodies. Target
 the same release train as #3217 if possible; they compound.
+
+## 6. Status of related PRs
+
+The experiment was intentionally throwaway (no tests/version-negotiation/docs,
+per the issue). Its codebase-change PR has been **closed** — everything it
+contained (the diff, the numbers, the problems, the production plan) is
+preserved in this document. Production work will be a fresh PR per §5.
+
+| PR | Branch | Status | Disposition |
+|---|---|---|---|
+| [#3293](https://github.com/shakacode/react_on_rails/pull/3293) | `experiment/3280-json-render-body` | **closed** | Experiment code (9-line `render_code` flip + instrumentation). Superseded by this plan; not for merge. Diff preserved verbatim in §1. |
+| [#3298](https://github.com/shakacode/react_on_rails/pull/3298) | `docs/3280-json-render-body-plan` (vs `upcoming-v16.3.0`) | closed | Earlier draft of this doc; relocated and re-targeted to `main`. |
+| this PR | `docs/3280-json-render-body-plan` (vs `main`) | open | This document. The only #3280 PR that stays open. |
+
+[#3217](https://github.com/shakacode/react_on_rails/pull/3217) (TCP_NODELAY)
+is a separate issue's PR, orthogonal to #3280 (§2.6) — **left untouched**.