Deduplicate SemanticLabel: remove Human, consolidate to Person

[Copilot]

SemanticLabel had two overlapping variants (Human and Person) representing the same concept. Human is removed; "human" string inputs are now remapped to SemanticLabel::Person.

Changes

• src/render/semantic.rs: Removed Human variant from SemanticLabel enum; dropped its arms from color() and as_str(); mapped "human" in from_label() to SemanticLabel::Person
• src/scene/semantic_room.rs: Updated human entity spawning to use SemanticLabel::Person

// Before
SemanticLabel::Human  // duplicate variant with same semantic meaning as Person

// After — "human" input strings still parse correctly
SemanticLabel::from_label("human") // => Some(SemanticLabel::Person)

The "human" mesh type name used in ZeroversePrimitives::Mesh("human") (burn_human mesh pipeline) is intentionally unchanged.

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