feat(rendering): Vendor Bevy rendering crates (Phase 1 complete)

Closes #6, #7, #8, #9, #10
Refs #2, #122

Vendored bevy_render, bevy_core_pipeline, and bevy_pbr from Bevy v0.17.2
(commit 566358363126dd69f6e457e47f306c68f8041d2a) into libmarathon.

- ~51K LOC vendored to crates/libmarathon/src/render/
- Merged bevy_render_macros into crates/macros/
- Fixed 773→0 compilation errors
- Updated dependencies (encase 0.10→0.11, added 4 new deps)
- Removed bevy_render/pbr/core_pipeline from app Cargo features

All builds passing, macOS smoke test successful.

Signed-off-by: Sienna Meridian Satterwhite <sienna@r3t.io>

crates/libmarathon/src/render/alpha.rs

@@ -0,0 +1,62 @@
+use bevy_reflect::{std_traits::ReflectDefault, Reflect};
+
+// TODO: add discussion about performance.
+/// Sets how a material's base color alpha channel is used for transparency.
+#[derive(Debug, Default, Reflect, Copy, Clone, PartialEq)]
+#[reflect(Default, Debug, Clone)]
+pub enum AlphaMode {
+    /// Base color alpha values are overridden to be fully opaque (1.0).
+    #[default]
+    Opaque,
+    /// Reduce transparency to fully opaque or fully transparent
+    /// based on a threshold.
+    ///
+    /// Compares the base color alpha value to the specified threshold.
+    /// If the value is below the threshold,
+    /// considers the color to be fully transparent (alpha is set to 0.0).
+    /// If it is equal to or above the threshold,
+    /// considers the color to be fully opaque (alpha is set to 1.0).
+    Mask(f32),
+    /// The base color alpha value defines the opacity of the color.
+    /// Standard alpha-blending is used to blend the fragment's color
+    /// with the color behind it.
+    Blend,
+    /// Similar to [`AlphaMode::Blend`], however assumes RGB channel values are
+    /// [premultiplied](https://en.wikipedia.org/wiki/Alpha_compositing#Straight_versus_premultiplied).
+    ///
+    /// For otherwise constant RGB values, behaves more like [`AlphaMode::Blend`] for
+    /// alpha values closer to 1.0, and more like [`AlphaMode::Add`] for
+    /// alpha values closer to 0.0.
+    ///
+    /// Can be used to avoid “border” or “outline” artifacts that can occur
+    /// when using plain alpha-blended textures.
+    Premultiplied,
+    /// Spreads the fragment out over a hardware-dependent number of sample
+    /// locations proportional to the alpha value. This requires multisample
+    /// antialiasing; if MSAA isn't on, this is identical to
+    /// [`AlphaMode::Mask`] with a value of 0.5.
+    ///
+    /// Alpha to coverage provides improved performance and better visual
+    /// fidelity over [`AlphaMode::Blend`], as Bevy doesn't have to sort objects
+    /// when it's in use. It's especially useful for complex transparent objects
+    /// like foliage.
+    ///
+    /// [alpha to coverage]: https://en.wikipedia.org/wiki/Alpha_to_coverage
+    AlphaToCoverage,
+    /// Combines the color of the fragments with the colors behind them in an
+    /// additive process, (i.e. like light) producing lighter results.
+    ///
+    /// Black produces no effect. Alpha values can be used to modulate the result.
+    ///
+    /// Useful for effects like holograms, ghosts, lasers and other energy beams.
+    Add,
+    /// Combines the color of the fragments with the colors behind them in a
+    /// multiplicative process, (i.e. like pigments) producing darker results.
+    ///
+    /// White produces no effect. Alpha values can be used to modulate the result.
+    ///
+    /// Useful for effects like stained glass, window tint film and some colored liquids.
+    Multiply,
+}
+
+impl Eq for AlphaMode {}