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.

site/src/content/docs/guides/viewer.md

@@ -107,6 +107,16 @@ It is honest about what it leaves out. Links carrying only a handful of referenc
 
 The map opens on your project's source directory. The picker switches to any other top-level folder or the whole repository, the checkbox brings test modules in, and `?depth=2` in the address splits a large folder into its sub-folders — the useful setting on a monorepo. What you are looking at lives in the URL, so the view is shareable.
 
+## Take the picture with you
+
+The flow strip and the map both carry **Copy image** and **Download SVG**.
+
+Copy image puts a PNG on the clipboard, ready to paste into a pull-request comment or a chat — the fastest way to say "this is what your change touches" without asking anyone to install something. Download SVG saves a file for a README: it is real text rather than a bitmap, so it stays sharp at any size and the symbol names in it are selectable and searchable.
+
+Both render the **light** theme whatever you are reading in, because the image is going to be read on somebody else's screen. Both carry a caption saying what the picture is — the path, or the root and how many modules — and a small CodeGraph mark in the corner. What you export is exactly what is on screen: the same hops, the same dashed dynamic-dispatch links, the same modules dimmed or brought forward by your selection, the same links hidden for being thin.
+
+An eight-hop strip comes out around half a megabyte, well inside what GitHub accepts inline.
+
 ## Options
 
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