opentui: Node 26 onboarding — scoped .node-version, engines floor, README setup guide

Pins Node 26.3 to ui-opentui/ only (fnm/mise auto-switch on cd; leaving
the directory restores whatever the dev had — no global default change).
engines.node >= 26.3 makes a wrong-Node npm ci warn. README covers
install paths (fnm/mise/nvm/absolute-binary), the ABI-locked
node_modules gotcha, and build/run commands.

ui-opentui/.node-version

@@ -0,0 +1 @@
+26.3

ui-opentui/README.md

@@ -0,0 +1,53 @@
+# ui-opentui — native OpenTUI engine for Hermes
+
+Solid + `@opentui/core` over Node FFI. Ink (`ui-tui/`) is the shipping default;
+this is the experimental engine (draft PR #42922).
+
+## Node 26 setup (required; will not touch your other projects)
+
+This package needs **Node ≥ 26.3** (`--experimental-ffi` floor). Everything
+else on this machine/repo can keep whatever Node it already uses — pin 26 to
+this directory only:
+
+```sh
+# 1. install fnm (skip if you have it; nvm/mise work too — see below)
+curl -fsSL https://fnm.vercel.app/install | bash
+# add to ~/.zshrc (or bashrc): eval "$(fnm env --use-on-cd --shell zsh)"
+
+# 2. install Node 26 SIDE BY SIDE (does NOT change your default)
+fnm install 26
+
+# 3. done — this directory has a .node-version (26.3), so `cd ui-opentui`
+#    auto-switches to 26 and leaving switches back. Do NOT run `fnm default 26`.
+node -v   # v26.x here; your old version everywhere else
+```
+
+No shell integration wanted (CI, scripts, one-off): `fnm exec --using 26 -- node ...`
+or invoke the absolute binary (`~/.local/share/fnm/node-versions/v26.*/installation/bin/node`).
+mise users: `mise use node@26` in this directory. nvm users: `nvm install 26`,
+plus an `.nvmrc` shim (`echo 26 > .nvmrc`) if you rely on auto-switching.
+
+### Gotchas
+
+- **Native modules are ABI-locked.** A `node_modules` installed under Node
+  20/22 will not load under 26 (and vice versa) — run `npm ci` (or
+  `npm rebuild`) after switching versions. Same applies to `bench/`'s node-pty.
+- **Global npm packages don't follow** between versions (per-version prefix);
+  reinstall the few you need, or don't use globals.
+- **Editor terminals** (Zed/VS Code) need the `fnm env` line in your shell rc;
+  the `.node-version` auto-switch then covers any shell that cd's here.
+- **Never run this package with bun** — the FFI seam and the Solid/JSX build
+  are Node-path only here.
+- `package.json` declares `engines.node >= 26.3`, so a wrong-Node `npm ci`
+  warns immediately.
+
+## Build & run
+
+```sh
+node scripts/build.mjs
+HERMES_TUI_MOUSE=1 node --experimental-ffi --no-warnings dist/main.js
+```
+
+Gates: `npm run check` (typecheck + lint + tests). Memory/perf benchmarks live
+in `../bench/` (see its README). Transcript windowing (memory architecture) is
+documented in `../docs/plans/opentui-transcript-windowing.md`.

ui-opentui/package.json

@@ -3,6 +3,9 @@
   "version": "0.0.0",
   "private": true,
   "type": "module",
+  "engines": {
+    "node": ">=26.3"
+  },
   "description": "Native OpenTUI engine for Hermes (Solid + Effect-at-boundary, from scratch). Ink (ui-tui/) stays the shipping default.",
   "scripts": {
     "type-check": "tsc --noEmit",