feat(wasm-client,web): the DOM renderer for the browser grid (improvise-cr3)

The last blocker of improvise-1ey. Mostly assembly: `Client` already had the
fold, the key classifier and `set_cell`, `CellDisplay::id` already said what a
cell is (ADR 002), and `flatten_cat_tree` already owned the category panel's
flattening. What was missing was the three joints between them.

`render.rs` is the one that matters. The DOM half is JavaScript and JavaScript
here is untested, so every question with a *silent* wrong answer is answered
in Rust under test and crosses as decided data: `editable` is a fact about the
`CellId` variant (Record only — an Aggregate is a drill target and `Drill` is
not on the wire yet, and `None` is "no projected cell", not "aggregate"), the
selection is placed by subtracting the scroll offset because `GridCache` is
viewport-indexed while the cursor is not, and the category panel is
`flatten_cat_tree` rather than a second walk of the cache.

`socket.rs` frames what `improvise-ws-server` actually speaks, which is not
what `wire.rs` speaks: it has `Viewport` (the handshake — a snapshot built at
extent zero is an empty grid), it batches `Commands`, and it has no `Key`
variant. The two enums are duplicated because a native crate cannot be a wasm
dependency; `improvise-ws-server` is a dev-dependency so the compatibility is
checked through its own types, and a companion test proves the check is
discriminating on the variant names that would drift.

`web/` is the shell, and it is thin on purpose: four string calls in and out,
no protocol knowledge, no framework.

Gates: 1047 passing (was 1031), clippy and fmt clean, and
`cargo check -p improvise-wasm-client --target wasm32-unknown-unknown` clean.
Verified end to end headlessly — the wasm-pack artifact driven from node over
a real websocket to a real `improvise-ws-server`: 1,200 read, "98765" typed,
"98,765" projected back. The DOM painting itself is unverified; there is no
headless browser here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EoW5BB87ZZdm96SvBTWawH

web/index.html

@@ -0,0 +1,44 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <title>improvise</title>
+    <link rel="stylesheet" href="./style.css" />
+  </head>
+  <body>
+    <!--
+      The browser shell for improvise-ws-server (improvise-cr3).
+
+      Deliberately three elements and no framework. Everything that could be
+      *wrong* — what a cell is, whether it may be typed into, where the
+      selection sits, how the category tree flattens — is decided in Rust and
+      arrives already decided (see `render.rs`). This page paints it.
+
+      Build the wasm module into ./pkg first:
+
+          wasm-pack build crates/improvise-wasm-client --target web --out-dir ../../web/pkg
+
+      then serve this directory over http (a module script will not load from
+      file://) and run the server beside it:
+
+          improvise-ws-server bank-info.improv --port 8787
+          python3 -m http.server 8080 --directory web
+
+      The socket address can be overridden with ?ws=ws://host:port.
+    -->
+    <header>
+      <h1>improvise</h1>
+      <span id="view-name"></span>
+      <span id="mode"></span>
+      <span id="status" role="status"></span>
+    </header>
+
+    <main>
+      <aside id="categories" aria-label="Categories"></aside>
+      <div id="grid" tabindex="0" aria-label="Grid"></div>
+    </main>
+
+    <script type="module" src="./app.js"></script>
+  </body>
+</html>