feat(ui): copy the flow or the map as an image, for a PR comment or a README (CG-55)

"Copy image" and "Download SVG" on the Flow strip's header and in the Map's
side panel. The image is the distribution loop: a flow pasted into a review, a
map pasted into a README, read by somebody with no viewer open.

The exporter serialises the LAYOUT OBJECT rather than scraping the DOM — no
html-to-image, no foreignObject, no new dependency. buildFlowLayout and
buildMapLayout already compute every rectangle, port and curve before a
component renders, so the image and the screen come from one piece of
arithmetic and cannot drift apart, and the whole exporter is a pure function a
test runs with no browser. Output is presentation-only SVG (rect, line, path,
polygon, text, tspan, clipPath) — no script, no external reference, no data:
URL — which is what GitHub's sanitiser accepts in a README.

Light theme is forced whatever the viewer is set to: a dark strip on GitHub's
white comment background reads as a mistake, not a preference. 24px of paper
around the drawing, a caption naming the path or the root at the bottom left,
a CodeGraph mark at the bottom right.

Fonts travel as family stacks, not bytes (spec). An SVG loaded as an image may
not fetch a webfont, so a raster falls back to the platform's own monospace —
every fallback in the stack advances at ~0.6em like IBM Plex Mono, so the code
grid survives and only the letterforms change. Text is truncated
arithmetically with an ellipsis and clipped as well, so a wider fallback
cannot spill a source line out of a card.

`scale` multiplies only the root width/height while the viewBox stays in CSS
pixels, so the raster draws an image whose intrinsic size is already 2x
instead of upscaling a 1x bitmap. The clipboard write uses the ClipboardItem
promise form (Safari discards the gesture across an await) and falls back to
downloading the PNG, saying which happened rather than claiming a copy it did
not make.

Measured on this repo: execute -> rowToFileRecord (8 hops) exports 3690x253
CSS px, 491 kB PNG at 2x / 38 kB SVG; the 16-module map reproduces the canvas
exactly — 16 boxes, 52 links, 9 layer rules, both band labels, and with
src/index.ts selected 15 links and 4 dimmed boxes.

ui/src/views/FlowView.svelte

@@ -19,6 +19,8 @@
   import FlowCard from '../components/flow/FlowCard.svelte';
   import FlowLink from '../components/flow/FlowLink.svelte';
   import FlowEndCap from '../components/flow/FlowEndCap.svelte';
+  import ExportButtons from '../components/ExportButtons.svelte';
+  import { exportFilename, flowSvg } from '../lib/export-svg';
   import { fetchFlow, type WireFlow, type WireFlowPayload } from '../lib/api';
   import { live } from '../lib/live.svelte';
   import { navigate, symbolHref } from '../lib/router.svelte';
@@ -223,6 +225,30 @@
     }
     return 'The longest call path among the symbols you named, the same one codegraph_explore leads with.';
   }
+
+  /**
+   * The strip as it stands, for a PR comment or a README.
+   *
+   * Built from `layout` — the same object the canvas is drawing — so the image
+   * cannot say something the screen does not. The caption names the path,
+   * because an image pasted into a review has lost the header that did.
+   */
+  const exportLabel = $derived(
+    showAll && flows.length > 1
+      ? `all ${flows.length} paths`
+      : (activeFlow?.label ?? 'flow')
+  );
+
+  function buildSvg(scale: number): string {
+    if (layout === null) throw new Error('There is no strip to export yet.');
+    const hops = activeFlow?.hops.length ?? 0;
+    return flowSvg(layout, {
+      scale,
+      activeFlowId: picked,
+      showAll,
+      caption: showAll ? exportLabel : `${exportLabel}${hops > 1 ? ` · ${hops} hops` : ''}`,
+    });
+  }
 </script>
 
 <div class="flowview">
@@ -251,6 +277,9 @@
     {#if payload}
       <p class="note">{note(payload)}</p>
     {/if}
+    {#if layout !== null}
+      <ExportButtons build={buildSvg} filename={exportFilename('flow', exportLabel)} />
+    {/if}
   </header>
 
   <div class="fstage">