chore(demo): updates docs around local and vendored switching

Author: grega

docs/README.md

@@ -17,10 +17,11 @@ The demo shows how the library transforms common Python error traces (from both
 - **KeyError**: Dictionary key not found
 - **ZeroDivisionError**: Division by zero
 
-Each example shows:
-1. The original Python traceback
-2. The code that caused the error
-3. The friendly explanation generated by the library
+The demo renders a full-width table with:
+1. A linkable row id for each example (URL hash)
+2. The example title and runtime
+3. The original code and traceback
+4. The friendly explanation generated by the library
 
 ### Usage
 
@@ -43,9 +44,16 @@ Then in the root directory of the project:
 
 2. Navigate to `http://localhost:8000/docs`
 
-## Local demo
+### Local source switching (development only)
 
-The `local-demo.html` file in this directory is a copy of the hosted demo that can be run locally, ie. against the local build of the library. Useful for development and testing.
+When running locally, the demo shows a source switch bar at the top:
+
+- default: vendored build (`docs/vendor/...`)
+- local mode: local build (`dist/index.browser.js`) via `?local=true`
+
+Local mode can be used to test changes to the library on-the-fly, as long as you have a local build running (eg. via `npm run dev:build`).
+
+On hosted environments (including GitHub Pages), the source switch UI is hidden and the demo always uses the vendored build.
 
 ### Usage
 
@@ -63,5 +71,7 @@ The `local-demo.html` file in this directory is a copy of the hosted demo that c
    npx http-server -p 8000
    ```
 
-3. Navigate to `http://localhost:8000/docs/local-demo.html`
+3. Navigate to one of:
+   - `http://localhost:8000/docs` (vendored build)
+   - `http://localhost:8000/docs?local=true` (local `dist/` build)