fix(chrome): clip diagram snapshots against the resized layout

[rapatao]

Follow-up to #56: large diagrams could still come out with the bottom cut, and with part of the surrounding page text baked into the top of the image.

Two causes

Clip measured against a stale layout. Emulation.setDeviceMetricsOverride is applied asynchronously, so measuring the diagram straight after resizing the viewport could read the pre-resize layout. Text above the diagram re-wraps when the viewport width changes, moving the diagram up or down; the clip then landed off it, pulling in surrounding page text at one edge and losing part of the diagram at the other. measureAfterResize now waits for window.innerWidth to report the new width before measuring.

Capture surface limit. Anything past the 4096 CSS px viewport cap relied on CaptureBeyondViewport, and past roughly 16384 px the browser stops growing its capture surface and returns an image cut off at the right and bottom. The viewport cap goes to 8192 CSS px, and captureScale lowers the device scale when 2x would exceed the surface limit, so an outsized diagram is captured whole (softer) rather than chopped.

Checks

case	result
200-node tall flowchart	83x15366, last node (Node 200) present — was 194x35710, past the surface limit
40-node wide flowchart	13947x81, Node 40 present at the right edge
text + 6x11 grid, html -flatten and docx	first node row at the top, last at the bottom, no page text bled in
testdata/complex.md (mermaid, d2, plantuml) to docx/epub/pdf	unchanged

New TestCaptureScale; go test ./... green.