[prakashUXtech]

What

PixelBuffer.glowing(color:radius:intensity:inner:) and PixelBuffer.shadowed(color:dx:dy:blur:inner:) in Sources/CoreRaster/Effects/AlphaEffects+Light.swift replace the FX-0 stubs, following the packet's "glowing / shadowed" pseudocode:

• Outer (glow, drop shadow): the layer's alpha is blurred, offset by (dx, dy) with +y down, tinted with the premultiplied colour, scaled by the intensity (glow) or by the colour's own alpha (shadow), then composited under the layer (src + paint * cover * (1 - layerAlpha)).
• Inner: the inverted alpha 1 - a is blurred and offset the same way, clipped by the layer's alpha, then composited over the layer (the same source-over that FX-5's inside stroke uses).
• A glow is the same operation with no offset.
• Blur mapping: the blur is FX-3's boxBlurred(radius: reach / 3), three box passes of radius r = reach / 3. This reaches exactly radius or blur pixels and approaches a Gaussian of sigma sqrt(r (r + 1)), about reach / 3. So radius → sigma ≈ radius / 3.
• Padding: the blur runs on an alpha mask padded with the outside value. That value is 0, or 1 for inner, so the area past the buffer reads as transparent. The padding is 2 px plus min(|offset|, reach + 3), so a shadow can pull in blur that spilled past the edge. The offset is compared as a Double before any Int conversion, so a huge offset just reads the outside value.
• Validation: each bad input throws RasterError.invalid with a message that names the field and its allowed range:
  • colour must be finite
  • dx and dy must be finite
  • radius and blur must be finite and in 0...1000 (the same cap as FX-3)
  • intensity must be finite and in 0...1, the range LayerEffect documents in W1Values
• Intensity 0 returns the buffer unchanged. A result component that is not finite also throws.

The glowAndShadowThrowNotBuilt stub case is removed from EffectStubTests.

Why

Effects-engine slice FX-alpha: glows and shadows composite under or over the layer by the published conventions, and FX-7's applying(_:) can now route .glow and .shadow here.

Packet ambiguities resolved

• Blur kernel: the packet says "Gaussian blurred". The existing gaussianBlur(sigma:) limits its kernel radius to twice the buffer's larger side, so a 1,000 px glow on a small layer would throw. Its direct convolution also costs O(radius) per pixel. Three box passes are the Gaussian approximation the packet already gives for FX-3, cost O(1) per pixel, and accept any reach. BlurFamily.swift is called, not edited.
• Edges: past the buffer reads as transparent, as the packet's Edges rule says for alpha effects. So an inner effect treats everything past the buffer as fully outside the shape, and a full-bleed layer gets an inner glow along the buffer's edges.
• Shadow opacity: shadowed has no intensity parameter. Its opacity is the premultiplied colour's alpha.
• Intensity range: 0...1, following the LayerEffect.glow doc in W1Values. Values outside it throw instead of being clamped.
• Offsets: there is no range cap, only a finiteness check. Anything far off the canvas reads the outside value.
• No shared helper was needed: AlphaEffects.swift is untouched.

Proof

• New suite Tests/CoreRasterTests/AlphaEffectLightTests.swift:
  • shadowMovesDown: the added alpha's centroid moves by (dx, dy) within 0.01 px. It checks fixed offsets and four seeded SplitMix64 offsets, including fractional ones.
  • innerStaysInside: on a disc and on the glyph mask, inner shadows and glows leave every alpha-0 pixel bit-exact and paint somewhere.
  • outerNeverCoversOpaque: an outer glow and a drop shadow leave every opaque pixel bit-exact.
  • intensityZeroIsIdentity: a glow at intensity 0 returns a buffer == to the input, inner and outer.
  • glowAndShadowStayInTheirRanges: a 1,000 px glow stays in range and keeps alpha at most 1.
  • badInputsThrowNamingTheField: each bad input throws, with the exact message.
  • hugeOffsetFallsOffTheCanvas: offsets of ±1e300 neither crash nor overflow.
• scripts/mutate --validate: OK, 669 anchors.
• scripts/mutate --run --plan CoreRaster.AlphaEffects: 2/2 caught. dy sign flipped is caught by shadowMovesDown; inner not clipped is caught by innerStaysInside.
• scripts/ci: CI: PASS, before and after rebasing on origin/dev.

Docs MCP: not used. There is no new platform or library API, only Swift stdlib arithmetic and the module's own helpers.

Part of #433

Belt-Lead: act-1a150bfbcf7-sfwy
Feature: effects-engine
Slice: FX-alpha
Mutate: 2/2 CoreRaster.AlphaEffects
Fast-Check: pass
Head: b2a691d

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[github-actions]

Fast lane: merged into dev (258 lines, 4 files, Fast-Check marker).