First piece of the layer-stream proxy. Resolves body_id -> source heightmap.png
read-only from systems.db (terrain_reference, mirroring CultureResolver's
pattern), searching roots mod-first over the base install. Explicit errors
(UnknownBody / NoTerrainReference / SourceMissing). v1 callers pass the base
root only; the search-order logic is proven with synthetic mod roots so the
mod-first seam is ready (Q-099 covers registering a mod's new bodies).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>