Add complete downstream API gate

api/API-COVERAGE.md

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+# Complete API gate coverage
+
+Generated by `python3 tools/check_api_coverage.py --write`; do not edit by hand.
+
+| Assembly | Catalog types | Mapped/exported/fixture types | Catalog members | Mapped/exported/fixture members |
+|---|---:|---:|---:|---:|
+| `LibreMetaverse` | 2,711 | 2,711 | 27,281 | 27,281 |
+| `LibreMetaverse.Imaging.Abstractions` | 3 | 3 | 20 | 20 |
+| `LibreMetaverse.Imaging.Skia` | 1 | 1 | 3 | 3 |
+| `LibreMetaverse.LslTools` | 164 | 164 | 768 | 768 |
+| `LibreMetaverse.PrimMesher` | 17 | 17 | 207 | 207 |
+| `LibreMetaverse.RLV` | 28 | 28 | 499 | 499 |
+| `LibreMetaverse.Rendering.MeshFoundry` | 1 | 1 | 14 | 14 |
+| `LibreMetaverse.Rendering.Simple` | 1 | 1 | 6 | 6 |
+| `LibreMetaverse.StructuredData` | 16 | 16 | 295 | 295 |
+| `LibreMetaverse.Types` | 45 | 45 | 942 | 942 |
+| `LibreMetaverse.Utilities` | 3 | 3 | 13 | 13 |
+| `LibreMetaverse.Voice.Vivox` | 64 | 64 | 531 | 531 |
+| `LibreMetaverse.Voice.WebRTC` | 12 | 12 | 210 | 210 |
+| **Total** | **3,066** | **3,066** | **30,789** | **30,789** |
+
+## Exclusions and boundary records
+
+- Public catalog type exclusions: **0**.
+- Public catalog member exclusions: **0**.
+- External signature types: **142** mapped to core/std, adopted cross-platform crates, or project-owned boundary types; their external member APIs are intentionally not copied.
+- Referenced non-public support traits: **2** mapped because public inheritance signatures require them.
+- Intentional delegate APM differences: **80**; every replacement is exported and compiled by the fixture.
+- Forbidden erased shim patterns: **0** (`ShimValue`, dynamic argument bags, invented variadics, or `Vec<Box<dyn Any>>` argument bags).
+
+The fixture is a standalone Rust workspace with path dependencies only. Building it requires neither .NET nor compiled LibreMetaverse assemblies.