v0.23.0

Alpha-aware software rasterisation. Offscreen buffers can now signal that their alpha channel matters downstream, and SwRenderer writes accumulate dst.a via non-premultiplied source-over instead of clobbering it to 255. Unblocks effect widgets (DropShadow and similar) that read the buffer's alpha as a silhouette mask.

Added

• AlphaMode enum + Texture::alpha_mode field (src/draw/texture.rs): Opaque (default — keeps the framebuffer path's "always write 255" behaviour) and Blend (accumulates dst.a via non-premultiplied source-over). AlphaMode is also re-exported from crate::draw::sw so existing use mirui::draw::sw::* callers see it alongside SwRenderer.
• SwRenderer::with_alpha_mode(mode) builder (src/draw/sw/mod.rs): sets the underlying target's mode. Default SwRenderer::new(target) is unchanged — Opaque.
• Four unit tests covering Opaque vs Blend semantics (src/draw/sw/mod.rs): default-mode regression, transparent-buffer first-hit alpha pass-through, two-fill source-over composition (±2 u8 rounding tolerance), and the src.a = 255 source-over identity short-circuit.

Changed

• Texture::blend_pixel_int reads self.alpha_mode (src/draw/texture.rs): the existing 4-arg signature is preserved (no caller migration needed). In Blend mode the destination alpha is composed via out.a = src.a + dst.a × (255 − src.a) / 255 instead of writing 255. The a == 255 short-circuit still writes 255 in both modes — that's the source-over identity at full source alpha.
• fill_axis_aligned falls back from the memcpy fast path when Blend mode + partial alpha (src/draw/sw/rect_fill.rs): fill_first_row_then_replicate would otherwise clobber the destination's alpha byte. The Opaque path and the fully-opaque source case in Blend mode keep the memcpy fast path.
• blit_1to1_argb_to_argb accumulates dst.a in Blend mode (src/draw/sw/blit_fast.rs): partial-alpha source pixels compose into the destination's alpha channel rather than overwriting it.
• render_system::try_draw_offscreen selects AlphaMode from OffscreenAlphaMode (src/widget/render_system.rs): an entity carrying OffscreenAlphaMode::clear_transparent() makes the inner SwRenderer use Blend. Other offscreen entities keep Opaque so the framebuffer's alpha gets pre-seeded as before.