fix(examples): mount www/ in Core-mode apps

[schloerke]

Stacked on #207 (base schloerke/example1-oldfaithful-hello). Split out of that PR to keep the two ideas separate: #207 rewrites the 01-hello example, this fixes a real bug it happened to surface.

The bug

page_react_html() reads index.html but does not serve the files index.html references. Shiny Express mounts the app directory's www/ at / automatically:

# shiny/express/_run.py
www_dir = file.parent / "www"
if www_dir.is_dir():
    app_opts["static_assets"] = {"/": www_dir}

Core's App() has no equivalent — static_assets defaults to None. So every Core app in this repo served / fine and then 404'd on its own scripts and stylesheets.

This is not a path-resolution issue (page_react_html() found index.html correctly all along), which is why it presented as "the page loads but nothing renders."

What was broken

Both Core-mode examples, and only those:

• examples/01-hello/app-core.py — 404 on /app.js and /main.css.
• examples/10-bookmarking/app.py — same cause, worse symptom: its entire client bundle 404'd, so it rendered a completely blank page.

Both now pass static_assets={"/": Path(__file__).parent / "www"}.

I swept all ten examples to confirm the blast radius. Eight are Express (set_react_page()) and unaffected. The remaining 404s are unrelated and expected: 03-columns-shadcn / 04-shadcn need npm run build (their bundles are gitignored) and 09-hmr gets www/app.js from the Vite dev server. Proof those are not this bug: www/index.html lives inside the mounted directory, and curl /index.html returns 200 on both — the mount is live, the bundles just aren't built.

Docs

The trap is invisible until you run the app, so it's documented where a Core user will actually meet it:

• a "Serving the client's files" section in page_react_html()'s docstring, with the working snippet
• the Core snippet in pkg-py/README.md, which previously showed App(page_react_html(...), server) — i.e. the broken form

R needs no counterpart: shiny::runApp() serves www/ next to app.R automatically. That is exactly why 01-hello's app.R worked all along and masked the Python-side gap.

Tests

None added — this is docs-and-examples only, with no package behavior change (page_react_html() is untouched apart from its docstring), so there is nothing new to assert.

Worth considering as follow-ups, happy to file either:

1. A CI smoke test over examples/ — boot each app, assert / and every src/href in its index.html returns 200. This is how 10-bookmarking's blank page would have been caught instead of sitting broken.
2. A warning from page_react_html() when the resolved index.html's directory isn't among the app's static mounts. That is a real behavior change and would need its own tests.

Verification

• app-core.py: /, /app.js, /main.css all 200, and the full slider → histogram round trip drives correctly in a browser.
• 10-bookmarking: renders again, and bookmark restore still hydrates — ?_inputs_&txt="hi"&num=7 populates the inputs and the server echoes text='hi' num=7.
• Console clean apart from favicon.ico.
• make py-check-tests (74 passed), make py-check-types clean, ruff clean.

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