Implement native SimpleRenderer reference pipeline (#76)

crates/libremetaverse-rendering-simple/README.md

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+# LibreMetaverse simple renderer
+
+`libremetaverse-rendering-simple` is the small, deterministic reference
+pipeline for geometry that is already represented by a `Primitive` or a
+decoded sculpt map. It has no windowing, GPU, Skia, voice, RLV, or LSL
+dependency. The only geometry backend is the workspace's checked native
+`libremetaverse-prim-mesher` crate.
+
+## Supported inputs and output
+
+`SimpleRenderer` implements both its concrete C#-mapped methods and the shared
+`IRendering` trait. It supports:
+
+- linear and flexible paths, plus circular prim paths;
+- square, circular, half-circle, and all three triangular profiles;
+- profile/path cuts, hollow profiles, shear, taper, twist, skew, radius offset,
+  and revolutions at all four `DetailLevel` values;
+- plane, cylinder, sphere, and torus sculpt maps supplied as `ManagedImage`;
+- faceted output grouped by prim face, and flattened simple output that keeps
+  each face's independent vertex/normal/UV domain;
+- per-face texture-entry inheritance, including texture, legacy material, and
+  render-material UUIDs; and
+- default and planar texture transforms with repeat, offset, rotation, and
+  primitive scale.
+
+Positions are emitted in prim-local coordinates. Every face contains finite
+positions, normals, and UVs, in-range triangle indices, its local bounds and
+center, and its effective texture-entry face. Identical inputs produce
+byte-for-byte-identical floating-point and index sequences; the compatibility
+suite pins a golden high-detail box.
+
+## Bounds and errors
+
+Each face and flattened mesh is limited to the 16-bit index domain (65,536
+vertices), and aggregate vertex/index growth uses checked arithmetic before
+allocation or conversion. Non-finite construction data, malformed mesher
+domains, invalid indices, unsupported topology, and budget overflow return
+`Error::Rendering`. That error includes the source primitive UUID and a static
+operation context so callers can identify the object that failed. Invalid
+standalone texture-transform arguments return `Error::Argument` because that
+API has no primitive parameter from which to derive source context.
+
+## Deliberate boundary
+
+A sculpt whose type is `Mesh` names an encoded mesh asset rather than a sculpt
+height map. `SimpleRenderer` does not fetch or decode assets, so that input
+returns a typed `mesh sculpt requires a decoded mesh asset` error. Mesh-asset
+decoding, skin weights, secondary UVs, and rigged-mesh metadata belong to the
+separate MeshFoundry renderer. The simple renderer also performs no world
+transform, scene traversal, material shading, texture download, or drawing.
+
+Run the focused implementation gates with:
+
+```console
+python3 tools/check_milestone_10_issue_76.py
+CARGO_BUILD_JOBS=1 cargo test -p libremetaverse-compat-tests --test rendering_shims --locked -j1
+CARGO_BUILD_JOBS=1 cargo test -p libremetaverse-rendering-simple --locked -j1
+```