Implement optional Skia codec adapter (#41)

crates/libremetaverse-imaging-skia/README.md

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+# MetaCrate Skia adapter
+
+This crate implements the pinned `SkiaTextureCodec` behavior at a bounded,
+project-owned image boundary. The core `libremetaverse-imaging` crate never
+exposes a `skia-safe` type and never discovers or links Skia.
+
+## Features and formats
+
+The default feature set has no Skia dependency. `SkiaTextureCodec::decode`
+returns a typed `InvalidOperation` error in that configuration, while conversion
+from the checked project-owned `backend::SKBitmap` remains available for tests
+and callers that already own decoded pixels.
+
+Enable native decoding with:
+
+```sh
+cargo test -p libremetaverse-imaging-skia --features skia
+```
+
+The `skia` feature decodes the CPU codec formats supported by the pinned
+rust-skia release: BMP, GIF, ICO, JPEG, PNG, WBMP, and WebP. Input is buffered
+to at most 64 MiB. Dimensions are rejected before native pixel allocation when
+they exceed the core 16,777,216-pixel limit, and decoded storage uses checked
+strides and owned Rust buffers.
+
+The mapped C# adapter exposes decoding and `SKBitmap`-to-`ManagedImage`
+conversion only; it has no encoding or resize method. Encoding selection stays
+with the format-specific imaging APIs, and callers use `ManagedImage`'s checked
+resize methods after decoding. Premultiplied pixels are converted to straight
+alpha at this boundary. RGB565, BGRA8888, RGBA8888, RGBA/BGRA1010102, Gray8,
+Alpha8, row padding, and the reference byte-width fallback retain the pinned
+C# channel and rounding rules.
+
+## Binary cache and source builds
+
+`skia-safe` 0.99.0 downloads an official prebuilt Skia archive when the target
+and Cargo feature set match a published archive. MetaCrate deliberately selects
+the published WebP-capable feature sets:
+
+- Linux and Windows: JPEG, PDF, SVG, text layout, Vulkan, and WebP;
+- macOS: JPEG, PDF, SVG, text layout, and WebP.
+
+Vulkan is selected on Linux and Windows only to match the published CPU/WebP
+archive; this adapter does not create a GPU context or call a platform graphics
+API. The same feature is therefore usable on macOS, Linux, and Windows without
+leaking platform-specific behavior.
+
+Build prerequisites for the supported desktop targets are:
+
+- Linux: a C++ linker/runtime, `curl`, `pkg-config`, FreeType, and Fontconfig;
+- macOS: Xcode command-line tools and `curl`;
+- Windows MSVC: the Rust MSVC toolchain, Visual Studio C++ build tools, and
+  `curl`.
+
+The build script downloads cache archives from the `rust-skia/skia-binaries`
+GitHub releases. Pin or mirror `SKIA_BINARIES_URL` in offline/reproducible build
+environments. `FORCE_SKIA_BINARIES_DOWNLOAD=1` makes a missing archive fail
+instead of compiling Skia. If no archive matches and that variable is absent,
+rust-skia falls back to a source build, which additionally requires Python 3,
+Ninja, and an LLVM/Clang toolchain. `FORCE_SKIA_BUILD=1` selects that path
+explicitly.
+
+## Licensing and redistribution
+
+`skia-safe` and rust-skia's bindings are MIT licensed; the linked Skia library
+is BSD-3-Clause licensed. Official binary-cache archives contain compiled Skia.
+Products that redistribute the resulting native artifacts must preserve the
+applicable MIT and BSD notices and audit the exact archive they ship.