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/README.md

@@ -46,9 +46,11 @@ src/
   lib/flow-model.ts       the Flow strip's card/link geometry + the end cap — a DAG (pure)
   lib/filecode-model.ts   the whole-file view: fixed line height, arcs, paging (pure)
   lib/entry-model.ts      the entry-points panel: rows, file groups, flow arming (pure)
+  lib/export-svg.ts       the Flow strip and the Map as a standalone SVG (pure)
+  lib/export-image.ts     rasterising that SVG to PNG, clipboard and download
   lib/live.svelte.ts      /api/events: two counters every screen refreshes from
   lib/toast.svelte.ts     the one transient note ("Index updated · reloaded")
-  components/             TopBar, TrailBar, KindGlyph, DriftBanner, Toast, map/, flow/, symbol/, file/, entry/
+  components/             TopBar, TrailBar, KindGlyph, DriftBanner, Toast, ExportButtons, map/, flow/, symbol/, file/, entry/
   views/                  one component per route
 ```
 
@@ -56,6 +58,27 @@ Fonts (Archivo Variable, IBM Plex Mono) are vendored through `@fontsource*` and
 emitted into `dist/viewer/assets`: a local reader must work offline and must not
 announce the project to a font CDN.
 
+## Export
+
+The Flow strip's header and the Map's side panel carry **Copy image** (a PNG on
+the clipboard) and **Download SVG** (a file for a README). Both render the
+**light** theme whatever the viewer is set to — an image is read on somebody
+else's screen — with 24px of paper around the drawing, a caption naming the path
+or the root, and a "CodeGraph" mark in the corner.
+
+`export-svg.ts` **serialises the layout object**; it does not scrape the DOM.
+`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 disagree — and the exporter is a pure function
+that a test can run with no browser at all. The output is presentation-only SVG
+(no script, no `foreignObject`, no external reference), which is what GitHub
+will render in a README.
+
+Fonts travel as `font-family` stacks rather than embedded bytes. 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.
+
 ## Routes
 
 | hash | view |