[Phase 2B] Aquatic: Add underwater vegetation and coral foundation

[MichaelFisher1997]

Part of #616.

Scope

Add the underwater vegetation and coral foundation needed for warm ocean and related ocean biome variants.

Candidate blocks/features from #616:

• seagrass and tall_seagrass
• kelp
• coral blocks and coral fans
• placement/render support needed for underwater decoration

Dependencies

Depends on #623 and #624.

Acceptance Criteria

• Underwater plants/coral have block registry entries, textures/materials, and suitable render shapes.
• Worldgen can place these decorations with water/environment constraints through the decoration profile system.
• Warm ocean work can consume this without adding another decoration API.
• nix develop --command zig build test passes.

[github-actions]

This is a Feature Request within the Phase 2B aquatic biome work (parent issue #616).

Analysis

What this requires:

1. Block Registry Extension (modules/world-core/src/block_registry.zig):

   • New BlockType entries for seagrass, kelp, coral blocks, coral fans
   • Appropriate RenderShape (cross works for kelp; coral fans may need a new shape like .fan or .flat)
   • RenderPass.cutout for vegetation; may need special fluid interaction for underwater placement

2. VegetationProfile Expansion (modules/world-worldgen/src/biome_registry.zig:45-56):

   • Add density fields for underwater vegetation: seagrass_density, kelp_density, coral_density, seaweed_density
   • These would mirror existing fields like grass_density, cactus_density

3. Decoration System Updates (modules/world-worldgen/src/decoration_registry.zig):

   • SimpleDecoration already supports place_on and biomes constraints
   • The DecorationContext in decoration_provider.zig lacks water depth/presence info — currently only has surface_y and surface_block
   • Need to add water constraint checking in StandardDecorationProvider.decorate() so decorations only place at valid underwater depths

4. Biome Additions (dependent on [Phase 1A] Rendering: Add flat quad and tall cross shapes #623, [Phase 1B] Worldgen: Generalize biome decoration profiles #624):

   • warm_ocean and tropical biome definitions with appropriate climate ranges (high temperature + high humidity + low continentalness)

Already exists:

• is_underwater flag in ColumnData (terrain_shape_generator.zig:48)
• Ocean water/floor handling in surface_builder.zig
• Decoration provider infrastructure that can be extended

Scope assessment: Well-scoped for a single PR. Implementation is additive (new blocks, new density fields, expanded context). No breaking changes to existing APIs.

Block type list: The issue body has empty placeholder text (- and ) where specific candidate blocks should be listed. Would be helpful to confirm exact block types before implementation.

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