feat(trace-charts): ordinal x-axis default + zoom/pan/cursor-sync (viewer Tier-1)

The served trace chart's x-axis no longer plots against raw timestamps by
default — non-trading gaps (nights, weekends) left horizontal blanks that
dominated the view. The x spine is now ordinal by default: every recorded slot
sits at an even position, so gaps collapse and the signal fills the width. A
header toggle flips to continuous (real-time-proportional) spacing and back;
default ordinal honours the requested "x-axis should not be continuous", with
continuous as the one-click option.

Comfort, all on the read/render side (the engine and the Rust serve contract
are untouched — window.AURA_TRACES still carries the real xs, the ordinal
indices are derived client-side):
- ordinal mode keeps the axis honest: ticks and the legend value-readout map a
  collapsed index back to the real xs[i], never the index itself.
- wheel-zoom (x, cursor-anchored), drag-pan, double-click-reset on the plot
  (uPlot's default drag-box-zoom is disabled so drag pans).
- panels share a cursor.sync key, so the crosshair lines up across stacked
  panels.

Confined to chart-viewer.js (buildCharts gains an xMode param + a wheelZoomPlugin
factory; mount re-renders on the toggle) and render.rs (CHART_HEAD toggle button
+ a chart-only style). The two twin node guards (build/gaps) re-point to the
ordinal default in lockstep; a new x-mode guard pins ordinal-default / continuous
/ panel-sync / plugin wiring, and a label-honesty guard (gappy-traces fixture,
xs far from the index spine) pins that ordinal ticks report the real timestamp.

Verified: cargo test --workspace green (incl. the four chart guards + render
inline tests), clippy --all-targets -D warnings clean, e2e (aura run --trace +
aura chart) emits a self-contained page defaulting to ordinal with the toggle.

Spec docs/specs/0057, plan docs/plans/0057 (boss-signed; grounding-check PASS).
Derived-fork decision log in the issue comments.

closes #103

crates/aura-cli/src/render.rs

@@ -59,10 +59,13 @@ const GRAPH_HEAD: &str = r#"<header>
 "#;
 
 /// The chart page's `<title>` text and `<header>` line — describes what the chart
-/// view actually renders (timestamp-aligned trace series), not the graph viewer.
+/// view actually renders (timestamp-aligned trace series), not the graph viewer. The
+/// `#xmode-toggle` button is wired by `chart-viewer.js` (`mount`) to flip the x-axis
+/// between the ordinal (gap-collapsed) default and continuous (real-time) spacing.
 const CHART_HEAD: &str = r#"<header>
   <b>aura chart</b>
   <span class="sub">timestamp-aligned trace series</span>
+  <button id="xmode-toggle" type="button">x: collapsed (gaps off)</button>
 </header>
 <div id="stage"></div>
 <pre id="err"></pre>
@@ -76,6 +79,16 @@ const UPLOT_JS: &str = include_str!("../assets/uPlot.iife.min.js");
 const UPLOT_CSS: &str = include_str!("../assets/uPlot.min.css");
 const CHART_VIEWER_JS: &str = include_str!("../assets/chart-viewer.js");
 
+/// Chart-page-only styling (the `#xmode-toggle` button) — injected by
+/// `render_chart_html` after the vendored uPlot CSS, NOT into the shared `SHELL_HEAD`
+/// (the graph page has no toggle and must not inherit its rule).
+const CHART_CSS: &str = r#"
+#xmode-toggle { margin-left: auto; background: #1e1e2e; color: #cdd6f4;
+  border: 1px solid #313244; border-radius: 6px; padding: 4px 10px;
+  font-family: ui-monospace, monospace; font-size: 12px; cursor: pointer; }
+#xmode-toggle:hover { border-color: #89b4fa; color: #f5e0dc; }
+"#;
+
 /// Assemble the self-contained interactive graph viewer page for `root`.
 ///
 /// Order is load-bearing: the Graphviz-WASM and pan-zoom globals (`Viz`,
@@ -146,12 +159,13 @@ pub fn render_chart_html(data: &ChartData, mode: ChartMode) -> String {
     .to_string();
 
     let mut html = String::with_capacity(
-        SHELL_HEAD.len() + CHART_HEAD.len() + UPLOT_JS.len() + UPLOT_CSS.len() + CHART_VIEWER_JS.len() + traces.len() + 256,
+        SHELL_HEAD.len() + CHART_HEAD.len() + UPLOT_JS.len() + UPLOT_CSS.len() + CHART_CSS.len() + CHART_VIEWER_JS.len() + traces.len() + 256,
     );
     html.push_str(&SHELL_HEAD.replace("{title}", "aura chart"));
     html.push_str(CHART_HEAD);
     html.push_str("<style>");
     html.push_str(UPLOT_CSS);
+    html.push_str(CHART_CSS);
     html.push_str("</style>\n<script>");
     html.push_str(UPLOT_JS);
     html.push_str("</script>\n<script>window.AURA_TRACES = ");
@@ -194,6 +208,8 @@ mod tests {
             html.contains("i64: \"#f9e2af\"") && html.contains("timestamp: \"#cba6f7\""),
             "C4 four-colour palette missing"
         );
+        // the chart-only toggle control must not leak onto the graph page
+        assert!(!html.contains("xmode-toggle"), "chart toggle leaked onto the graph page");
     }
 
     fn sample_chart_data() -> ChartData {
@@ -229,6 +245,8 @@ mod tests {
         let html = render_chart_html(&sample_chart_data(), ChartMode::Overlay);
         assert!(html.contains("<title>aura chart</title>"), "chart page title not chart-specific");
         assert!(html.contains("<b>aura chart</b>"), "chart header label not chart-specific");
+        // the x-axis toggle control is part of the chart chrome
+        assert!(html.contains("id=\"xmode-toggle\""), "x-axis toggle control missing from the chart page");
         // none of the graph viewer's chrome leaked in
         assert!(!html.contains("aura graph"), "graph label leaked onto the chart page");
         assert!(!html.contains("hover · [+] expand · body drill"), "graph breadcrumb leaked onto the chart page");