feat(shaders): add simple single-pass Blur shader

[chiefcll]

Summary

• Adds a Blur shader for both WebGL and Canvas2D backends, intended primarily for nodes with an Image Texture.
• WebGL: single-pass 9-tap Gaussian-approximation kernel (1,2,1 / 2,4,2 / 1,2,1) / 16 — no render target, no second pass, fixed sample count for predictable cost on constrained devices.
• Canvas2D: delegates to the native ctx.filter = 'blur(Npx)' and relies on saveAndRestore: true so the renderer cleans up filter state automatically.
• Single amount prop (node-space pixels, default 4).

Files

• src/core/shaders/templates/BlurTemplate.ts — props + default
• src/core/shaders/webgl/Blur.ts — WebGL fragment shader
• src/core/shaders/canvas/Blur.ts — Canvas2D filter-based shader
• exports/{webgl,canvas}-shaders.ts — public re-exports
• examples/tests/shader-blur.ts — visual regression example at amounts [0, 2, 6, 12]
• src/core/shaders/templates/BlurTemplate.test.ts — unit tests

Tradeoffs

• Single-pass design: large amount values (>~8) lose smoothness because there are only 9 fixed taps. For heavier blur, a separable two-pass via RenderTexture would be needed — intentionally out of scope here per "simple/performant."
• Canvas2D blur depends on native ctx.filter (Chrome 53+). Browsers below the floor will silently no-op rather than emulate in JS.

Test plan

• pnpm build — clean
• pnpm test (BlurTemplate) — passing
• pnpm test:visual:update then review snapshot for shader-blur before merging
• Manually verify on an Image Texture node in pnpm start

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