fix: expose anchor size CSS variables

[draedful]

Summary

• provide default --graph-block-anchor-width and --graph-block-anchor-height values on the graph wrapper
• consume the size variables without fallback declarations while preserving CSS overrides
• document anchor sizing and add browser regression coverage for defaults and overrides

Root cause

Anchor coordinates were exposed as concrete CSS custom-property values, while width and height existed only as fallbacks inside var(...). As a result, consumers could not read default values from the custom properties and CSS processing could emit duplicate width and height declarations.

Impact

The rendered anchor size remains 16px × 16px by default. Consumers can continue overriding either dimension through the CSS cascade; no inline values are introduced.

Validation

• CI=1 npx playwright test e2e/tests/anchor/anchor-css-variables.spec.ts
• npm run typecheck
• npm run lint
• npm test -- --runInBand
• git diff --check

Fixes #329

Summary by Sourcery

Expose and default anchor size CSS variables while documenting and testing their usage.

Bug Fixes:

• Ensure anchor width and height CSS variables are properly exposed with default values on the graph wrapper to avoid duplicate declarations and allow consumers to read them.

Enhancements:

• Simplify anchor sizing CSS by relying directly on the exposed custom properties for width and height.

Documentation:

• Document how to configure anchor size via CSS variables in the React usage guide.

Tests:

• Add Playwright end-to-end tests to verify default anchor size variables and consumer overrides for width and height.

[sourcery-ai]

Reviewer's Guide

This PR centralizes default anchor sizing on the graph wrapper via CSS variables, updates the anchor styles to always consume those variables directly (no fallbacks), documents how to size anchors, and adds Playwright coverage to verify default and overridden anchor dimensions via CSS custom properties.

File-Level Changes

Change	Details	Files
Anchor component now relies solely on CSS custom properties for width/height instead of embedding fallback values.	Replaced var(...) usages for internal --width/--height with references to --graph-block-anchor-width and --graph-block-anchor-height without fallbacks. Updated width and height declarations on the anchor element to use --graph-block-anchor-width and --graph-block-anchor-height directly, removing inline fallback sizes.	src/react-components/Anchor.css
Graph wrapper now exposes default anchor size via CSS variables.	Defined --graph-block-anchor-width and --graph-block-anchor-height defaults (16px) on the .graph-wrapper container. Ensured layout-related styles on .graph-wrapper remain unchanged.	src/react-components/graph-canvas.css
Anchor sizing via CSS variables is documented and covered by e2e tests.	Extended usage docs to show overriding --graph-block-anchor-width and --graph-block-anchor-height in consumer CSS. Added Playwright tests to assert default anchor width/height custom property values and corresponding computed sizes. Added Playwright tests to assert that consumer overrides of anchor width/height via CSS custom properties propagate to computed styles.	docs/react/usage.md e2e/tests/anchor/anchor-css-variables.spec.ts

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#329	Expose CSS custom properties --graph-block-anchor-width and --graph-block-anchor-height with default values for anchor size so consumers can read and override them.	✅
#329	Use the anchor size CSS variables consistently in the anchor styles without redundant hardcoded width/height declarations.	✅

Possibly linked issues

• Missing css-variables of graph-block-anchor for width and height #329: The PR adds and documents the default anchor width/height CSS variables exactly as requested in the issue.

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