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/style.css

@@ -0,0 +1,141 @@
+/* Legibility only. Styling the browser client properly is not improvise-cr3. */
+
+:root {
+  --bg: #fbfaf7;
+  --fg: #1c1b19;
+  --line: #d8d4cc;
+  --accent: #2f6f4f;
+  --selected: #e8e2d4;
+  --error: #a8322d;
+  color-scheme: light dark;
+}
+
+@media (prefers-color-scheme: dark) {
+  :root {
+    --bg: #17181a;
+    --fg: #e7e4de;
+    --line: #35373b;
+    --accent: #7fc4a0;
+    --selected: #2c2f33;
+    --error: #e2827c;
+  }
+}
+
+* {
+  box-sizing: border-box;
+}
+
+body {
+  margin: 0;
+  height: 100vh;
+  display: flex;
+  flex-direction: column;
+  background: var(--bg);
+  color: var(--fg);
+  font: 14px/1.4 ui-monospace, SFMono-Regular, Menlo, monospace;
+}
+
+header {
+  display: flex;
+  gap: 1rem;
+  align-items: baseline;
+  padding: 0.5rem 1rem;
+  border-bottom: 1px solid var(--line);
+}
+
+header h1 {
+  margin: 0;
+  font-size: 1rem;
+  color: var(--accent);
+}
+
+#mode {
+  color: var(--accent);
+}
+
+#status {
+  margin-left: auto;
+  opacity: 0.75;
+}
+
+main {
+  flex: 1;
+  display: flex;
+  min-height: 0;
+}
+
+#categories {
+  width: 15rem;
+  flex: none;
+  overflow: auto;
+  padding: 0.5rem;
+  border-right: 1px solid var(--line);
+}
+
+#categories ul {
+  margin: 0;
+  padding: 0;
+  list-style: none;
+}
+
+#categories .item {
+  padding-left: 1.5rem;
+  opacity: 0.85;
+}
+
+#categories .selected,
+td.selected {
+  background: var(--selected);
+  outline: 1px solid var(--accent);
+}
+
+#grid {
+  flex: 1;
+  overflow: auto;
+  padding: 0.5rem;
+}
+
+table {
+  border-collapse: collapse;
+}
+
+th,
+td {
+  border: 1px solid var(--line);
+  padding: 0.25rem 0.5rem;
+  min-width: 6rem;
+  height: 1.75rem;
+  text-align: left;
+  font-weight: normal;
+}
+
+th {
+  background: color-mix(in srgb, var(--line) 35%, transparent);
+  font-weight: 600;
+}
+
+td.numeric {
+  text-align: right;
+}
+
+td.error {
+  color: var(--error);
+}
+
+/* The only affordance that matters: a cell the server said is settable. */
+td.editable {
+  cursor: text;
+}
+
+td:not(.editable) {
+  opacity: 0.7;
+}
+
+td input {
+  width: 100%;
+  border: none;
+  padding: 0;
+  font: inherit;
+  color: inherit;
+  background: transparent;
+}