MDL-87987 [docs] add React Profiler guide for dev-mode mounting

docs/guides/javascript/react/profiler.md

@@ -0,0 +1,65 @@
+---
+title: React Profiler
+tags:
+  - react
+  - javascript
+  - moodle
+description: How to use Moodle's React profiling and dev-mode bundle infrastructure, including programmatic mounting and the Profiler HOC.
+---
+
+## How it works
+
+Developer mode is signalled by `jsrev = -1`, which is set when **Site administration → Development → Debugging → Cache JavaScript** is disabled. This is the same convention used by Moodle's existing JS and CSS loaders.
+
+When `jsrev === -1`:
+
+- **PHP** — `import_map::resolve_react_dev_path()` substitutes `client.development.js` (the unminified React DOM profiling build) for `client.js` at file-serve time, giving cleaner stack traces and full React warnings.
+- **Browser** — `isProfilerEnabled()` returns `true`. `mountReactApp()` automatically wraps every component tree in a React [`<Profiler>`](https://react.dev/reference/react/Profiler) and logs render timings to the console.
+
+## Writing a new React component
+
+Place source files under `<component>/js/esm/src/` and build output to `<component>/js/esm/build/`. The import map resolves `@moodle/lms/<component>/<module>` to `<componentdir>/js/esm/build/<module>.js`.
+
+:::caution
+
+The module must have a **default-exported** React function component:
+
+:::
+
+```ts
+// mod_book/js/esm/src/viewer.ts
+export default function Viewer({ title }: { title: string }) {
+    return <h1>{title}</h1>;
+}
+```
+
+## Mounting programmatically
+
+Most components should be mounted via the Mustache helper, which handles auto-initialisation automatically — see [Mustache helper](./reactautoinit.md#mustache-helper-react). Use `core/mount` directly when you need to mount from TypeScript/JavaScript — for example, when the mount point is created dynamically, when you need the unmount handle, or when mounting inside an existing JS module lifecycle.
+
+```ts
+import { mountReactApp, unmountReactApp } from "@moodle/lms/core/mount";
+
+const unmount = mountReactApp(container, MyComponent, props, { id: "my-app" });
+
+// Unmount when done:
+unmount();
+// or, if you only have the container element:
+unmountReactApp(container);
+```
+
+`mountReactApp` wraps the component in `<Profiler>` automatically when dev mode is active — no extra code needed.
+
+## Console output in dev mode
+
+When profiling is active the browser console shows:
+
+| Output | Meaning |
+|--------|---------|
+| `[mount] MyComponent - 2.34ms` | Collapsed group for each render |
+| `console.warn` Slow render | Actual render time exceeded 16 ms (60 fps budget) |
+| `console.error` Very slow render | Actual render time exceeded 50 ms |
+| `[react_autoinit] Initializing...` | Auto-init started |
+| `[react_autoinit] Found N component(s) to mount` | Components detected in DOM |
+| `[react_autoinit] Mounted via default: <specifier>` | Successful mount |
+| `[react_autoinit] Unmounted: <specifier>` | Clean unmount after DOM removal |

project-words.txt

@@ -361,3 +361,4 @@ savechanges
 hideif
 formslib
 autoinit
+unminified