feat(viz): rework the benchmark report's controls, axes and export

[4D0R]

What

Reworks the generated HTML report (benchmark/visualize.go). No change to
benchmark execution, the JSONL schema, or the CLI.

Axes

• The latency axis follows the rolling percentile lines that are drawn,
  rather than raw per-request extremes, and stays put when layers are
  toggled — ticking a layer reveals a line instead of rescaling the chart.
• Cache-mix bands reserve their own vertical space, so another layer can no
  longer draw through them.
• Totals get a right-hand axis in token units, present only while the layer
  is shown, replacing an unlabelled 0-1 fraction that shared the latency
  scale.

Requests layer

• Draws a p10-p90 spread band per arm where individual dots would overplot,
  falling back to dots once the view is zoomed in enough to read them. The
  threshold is evaluated once per chart, so arms cannot render in different
  styles at the same zoom.
• Dots no longer depend on the line toggles.

Controls

• Select All / Deselect All act on the plotted layers; variant selection
  moved into the filter modal beside the checkboxes it controls, where it
  previously drove a widget in another panel and appeared to do nothing.
• A single state-render pass keeps legend, modal and summary consistent.
• Labels renamed (Response Time / TTLT, no Show prefix).
• Errors and Totals now start unchecked, so a freshly opened report shows the
  latency lines without the ingest area over them.
• Removed the variant filter text box (the legend and the filter modal
  already cover variant visibility) and the per-tick x-axis value rows.

Help

• Custom tooltip replaces native title: styled, prompt, keyboard reachable,
  and not clipped by a scrolling panel. The affordance is a dotted underline
  on the label rather than an added glyph.

Zoom

• Esc exits a zoom. Double-click retained; Reset Zoom button removed.

Export

• CSV of per-request rows and of the summary, scoped to the current view
  (zoom window, hidden arms, filters), with a commented provenance header.
  Generated client side, so a report that has been copied elsewhere still
  yields its data without network access.

Testing

• gofmt, go vet, and the suite under -race.
• The emitted script is exercised by the Node-backed tests in
  benchmark/visualize_samples_test.go.
• Checked in a browser against a generated report: control state propagation,
  tooltip placement and keyboard paths, Esc precedence, axis stability
  across layer permutations, the band/dots threshold, and CSV row counts and
  scoping.

Notes

• benchmark/visualize_samples_test.go drops assertions for the removed
  x-axis-values control and updates two label strings.
  TestNoDatasetAxisRowsJS keeps its real assertion — that the cache-mix
  overlay must not grow the bottom margin — now that the bottom margin is a
  fixed constant.
• The CSV carries the fields embedded in the report, a subset of the JSONL;
  the header says so and points to the full CSV written alongside a run.