SVG <filter>: a multi-primitive graph

The classic drop-shadow-via-primitives idiom: feFlood + feComposite ("in") builds a solid shadow shape from the source's own alpha, feOffset displaces it, and feMerge layers it under the original. Each step names its own result (flood, shadowColor, offsetShadow) so a later primitive - including feMergeNode's own "in" - can look it up directly, not just chain to the immediately-previous step. (docs/supported-svg-features.md#filters)