T24f Agent: control and cost

HumanInputRequest asks a question with answers to pick and a line to
write one; CostBreakdown totals a session and splits it by part.
InterruptButton, TokenUsageChart, TraceViewer and EvalResultTable
point to chat::stop_button, a stacked BarChart, TimelineProfiler and
DataTable.

UsageBar shows amounts as its owner writes them. MemoryPanel and
HumanInputRequest share agent::line::send_line; the chapter's bool
callbacks take forms::OnFlag.

DataTable scrolls columns wider than its box sideways, pinned or not,
and leaves a plain wheel to the page; sortable headers are Tab stops.
TimelineProfiler fits its ruler to its lane, its spans are Tab stops,
and a picked span takes accent.

src/agent/line.rs

@@ -0,0 +1,58 @@
+use std::rc::Rc;
+
+use gpui::{App, Div, Entity, InteractiveElement, ParentElement, Styled, Window, div};
+
+use crate::{
+    buttons::Button,
+    forms::{Enter, Input, TextInput},
+};
+
+pub(super) type OnText = Rc<dyn Fn(&str, &mut Window, &mut App)>;
+
+/// Hands the field's words to `send` and empties it; blank words are kept out.
+fn send(
+    field: &Entity<TextInput>,
+    on_send: &OnText,
+    what: &str,
+    window: &mut Window,
+    cx: &mut App,
+) {
+    let text = field.read(cx).text().trim().to_string();
+    if text.is_empty() {
+        return;
+    }
+    log::info!("{what}: sent {} characters", text.len());
+    on_send(&text, window, cx);
+    field.update(cx, |input, cx| input.set_text("", cx));
+}
+
+/// The owner's field beside `button`: Enter or a press sends its words, and the button waits for some.
+pub(super) fn send_line(
+    field: &Entity<TextInput>,
+    button: Button,
+    on_send: OnText,
+    what: &'static str,
+    cx: &App,
+) -> Div {
+    let ready = !field.read(cx).text().trim().is_empty();
+    let (typed, pressed, enter) = (field.clone(), field.clone(), on_send.clone());
+    div()
+        .flex()
+        .items_center()
+        .gap_2()
+        .child(
+            div()
+                .flex_1()
+                .min_w_0()
+                .capture_action(move |_: &Enter, window, cx| {
+                    cx.stop_propagation();
+                    send(&typed, &enter, what, window, cx)
+                })
+                .child(Input::new(field)),
+        )
+        .child(
+            button
+                .disabled(!ready)
+                .on_click(move |_, window, cx| send(&pressed, &on_send, what, window, cx)),
+        )
+}