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.

tasks/ch21-30.md

@@ -201,12 +201,12 @@
 
 ## T24f Agent · Control and cost
 
-- [ ] InterruptButton
-- [ ] HumanInputRequest
-- [ ] TokenUsageChart
-- [ ] CostBreakdown
-- [ ] TraceViewer
-- [ ] EvalResultTable
+- [x] InterruptButton → `chat::stop_button`
+- [x] HumanInputRequest
+- [x] TokenUsageChart → `charts::BarChart` (stacked)
+- [x] CostBreakdown
+- [x] TraceViewer → `debug::TimelineProfiler` (a track per depth)
+- [x] EvalResultTable → `tables::DataTable` (Tag and Progress cells)
 
 ## T25 Generative — `src/generative`