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/build.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# Build the wasm bundle this page loads, then say how to run it.
+#
+# Needs wasm-pack (cargo install wasm-pack) and the wasm32-unknown-unknown
+# target. Nothing else in the repo depends on either — the `wasm` CI job only
+# runs `cargo check --target wasm32-unknown-unknown`, which does not need a
+# bindgen step.
+set -eu
+root=$(cd "$(dirname "$0")/.." && pwd)
+wasm-pack build "$root/crates/improvise-wasm-client" \
+  --target web --out-dir "$root/web/pkg"
+cat <<'MSG'
+
+Built. To run it, in two terminals:
+
+  improvise-ws-server your-model.improv --port 8787
+  python3 -m http.server 8080 --directory web
+
+then open http://127.0.0.1:8080/ (override the socket with ?ws=ws://host:port).
+MSG