fix(html): flatten diagrams to inline SVG, stop diagram overflow scrollbars

[rapatao]

Follow-up to #56 and #57, which chased the same symptom twice: a diagram snapshot cut at the right and bottom, then cut at the top and bottom. Both were the same underlying bet — measure a region of a live page and clip a screenshot to it — losing to scroll position, page reflow and browsers that ignore CaptureBeyondViewport.

Changes

HTML -flatten stops rasterizing. chrome.FlattenDiagrams (was Rasterize) now does what the PDF path does: the browser draws each diagram, the rendered inline SVG stays in the document, the mermaid library is stripped. Nothing is measured, clipped or snapshotted, so no diagram can come out cut; the result is resolution-independent and much smaller (a test doc went from 3.7 MB of PNG to 81 KB).

Consequence, documented in the README: Google Docs does not render inline SVG on import, so -f docx (PNG diagrams) is the route for that.

Diagram blocks no longer produce a scrollbar in PDF. pre.mermaid inherited pre{overflow:auto}, so a diagram wider than its box got a scroll region — painted into the printed PDF and clipping the diagram. Now:

pre.mermaid{...;overflow:visible}
pre.mermaid svg,div.d2 svg,div.plantuml svg{max-width:100%;height:auto}

The svg rule also stops a wide d2/plantuml diagram running off the page. Verified with a 3000px-wide SVG in a 900px viewport: clipped at the box edge before, scaled to fit after.

DOCX keeps a raster (Word needs one), but snapshotDiagram now pins the diagram position:fixed at the viewport origin, alone, scaled to fit a viewport sized to it. The clip is (0,0,w,h) — the same rectangle under every screenshot coordinate convention, wherever the page is scrolled, honoured flag or not.

Checks

case	result
text + 6x11 grid → docx	top row = node 0, bottom row = node 10
200-node flowchart → docx	94x16378, ends at Node 200
40-node wide flowchart → docx	16384x86, ends at Node 40
-flatten html	inline <svg>, no <script>, no data:image/png
testdata/complex.md → docx/epub/pdf	unchanged

New TestFlattenDiagramsKeepsRenderedSVG; go test ./... green. README, -flatten flag help and package docs updated to match.