Make MDL SVG rendering deterministic

[raphael]

Problem

mdl svg could start saving before the lazy graph mounted, then hide the browser error and time out while polling for a file. Automatic relationship labels could overlap nodes, and the CLI's implicit DOWN direction overrode each view's AutoLayout declaration.

Outcome

• The graph component now signals DOM readiness before headless layout begins. Layout and save failures remain visible in the editor and are returned to the CLI through an explicit browser completion status.
• Auto-generated relationship labels evaluate both sides and multiple positions on their owning segment, choosing the first collision-free placement while preserving manually moved label anchors.
• A view's declared rank direction is the default. -direction DOWN|UP|LEFT|RIGHT remains an explicit override; invalid values now fail instead of silently falling back.

The browser owns graph readiness and action sequencing, the save endpoint owns durable file creation, and the CLI owns timeout and diagnostic reporting. No model membership, relationship selection, or authored manual layout contract changes.

Compatibility

Callers that unintentionally relied on the old implicit top-to-bottom override will now receive their view's declared direction. They can preserve the old output with -direction DOWN.

Validation

• pnpm exec tsc --noEmit
• pnpm run build
• CHROME_BIN=\"/Applications/Google Chrome.app/Contents/MacOS/Google Chrome\" go test ./... -count=1
• Verified the regression fixture has three label/node collisions with v1.16.3 and none with this branch.
• Regenerated the Flows service diagram and measured zero node/label and peer-label collisions.

Review guide

Start with the automation status contract in Root.tsx and main.go, then review collision scoring in graph.ts and view-direction mapping in parseModel.ts. Browser-backed regressions are in svg_e2e_test.go.