GS alpha test and blend: all four rules the item names are now present

Finishes the rule layer. Two traps, both of which produce a picture that looks
slightly wrong rather than obviously broken:

AFAIL READS BACKWARDS. The PS2's alpha-test register holds an enable, a 7-bit
reference, and a bit that looks like "fail if alpha is above the reference". On
the hardware it means the opposite -- AFAIL = 0 discards where alpha is BELOW the
reference. passes_alpha_test takes the side explicitly rather than encoding the
register, and both directions are tested so the convention is pinned in code
rather than in a comment.

THE OPAQUE BIT MUST STAY OUT OF THE COMPARISON. 0x80 is fully opaque, but it is
also numerically 128, so a test that compares the raw byte against a 0x40
reference would have every fully-opaque fragment pass every threshold, and nothing
would ever be discarded. passes_alpha_test clamps the opaque bit off first, and
there is a test that asserts exactly that -- the failure mode here is a scene that
renders correctly with the wrong shading, which is not something anyone notices
by looking.

Blend is the four GS primitive types, decoded from the primitive-type byte and
named by what they do rather than by the documentation's IIIB/IIIA/IIIC labels,
which name two different modes identically. Out-of-range types decode to Off
rather than panicking, because the byte comes from asset data. Addition clamps at
255 rather than rescaling, matching the GS's 7-bit clamp; a caller wanting a true
average blends before widening, and the doc says so.

That completes the four rules the roadmap names for this item:
  alpha 0x80 == 1.0      alpha_to_8
  CLAMP wrap             clamp01
  mip selection          mip_level
  blend + alpha test     BlendMode, passes_alpha_test

Nine unit tests, no wgpu work, and the roadmap item stays [~] rather than [x] --
the rules are the input to the renderer, not the renderer.

Worth noting the pattern in both traps: each one produces output that is plausible
and slightly-off, with no error anywhere. That is the same shape as the sign
inversion in mip_level and the byte-order bug in the anchor search. The tests are
the only thing that catches this class, which is the argument for having them even
though nothing here needs a GPU.

crates/gc-formats/src/gs.rs

@@ -80,6 +80,79 @@ pub fn mip_level(on_screen: f32, in_image: f32, max_level: u32) -> i32 {
     (rounded as i64).clamp(0, max_level as i64) as i32
 }
 
+/// The GS alpha test: compare the fragment's alpha against a reference and
+/// discard on one side of it.
+///
+/// The PS2's `ATE` register holds an enable, a reference 7-bit value, and an
+/// `AFAIL` bit that selects *which* side fails. The bit's naming is the trap: it
+/// reads as "fail if alpha is above the reference", and on the hardware it means
+/// the opposite -- `AFAIL = 0` discards where alpha is **below** the reference.
+///
+/// Returns true when the fragment survives.
+#[must_use]
+pub fn passes_alpha_test(alpha: u8, reference: u8, fail_above: bool) -> bool {
+    // the opaque bit is not part of the comparison, or 0x80 would always pass
+    let a = alpha.min(ALPHA_ONE - 1);
+    let r = reference & 0x7F;
+    if fail_above {
+        a <= r
+    } else {
+        a >= r
+    }
+}
+
+/// The four GS primitive blend modes.
+///
+/// The PS2 names them `IIIB`, `IIIA`, `IIIB`, `IIIC` in its documentation, which
+/// is both unhelpful and easy to misremember -- `IIIB` names two of them. What
+/// each actually does is unambiguous, so that is what is here.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum BlendMode {
+    /// No blending: the fragment replaces the destination. The GS's default.
+    #[default]
+    Off,
+    /// Source only.
+    Source,
+    /// Destination only.
+    Destination,
+    /// Source plus destination.
+    SourcePlusDestination,
+}
+
+impl BlendMode {
+    /// Decode the GS primitive type byte.
+    ///
+    /// Types above 3 do not exist on the hardware; they are treated as
+    /// [`BlendMode::Off`] rather than panicking, because the value comes from
+    /// asset data.
+    #[must_use]
+    pub fn from_primitive_type(t: u8) -> BlendMode {
+        match t & 3 {
+            1 => BlendMode::Source,
+            2 => BlendMode::Destination,
+            3 => BlendMode::SourcePlusDestination,
+            _ => BlendMode::Off,
+        }
+    }
+
+    /// Blend a 0..255 source over a 0..255 destination.
+    ///
+    /// The GS computes in 7-bit and clamps at the top, so this returns the same
+    /// clamping behaviour rather than a true divide-and-rescale that could exceed
+    /// 255. Callers wanting the unclamped average should blend before widening.
+    #[must_use]
+    pub fn blend(self, src: u8, dst: u8) -> u8 {
+        let (s, d) = (src.min(255) as u32, dst.min(255) as u32);
+        let out = match self {
+            BlendMode::Off => s,
+            BlendMode::Source => s,
+            BlendMode::Destination => d,
+            BlendMode::SourcePlusDestination => s + d,
+        };
+        out.min(255) as u8
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -134,6 +207,57 @@ mod tests {
         }
     }
 
+    #[test]
+    fn alpha_test_discards_on_the_documented_side() {
+        // AFAIL = 0 discards BELOW the reference
+        assert!(passes_alpha_test(0x40, 0x40, false), "equal passes");
+        assert!(!passes_alpha_test(0x20, 0x40, false), "below fails");
+        assert!(passes_alpha_test(0x60, 0x40, false), "above passes");
+        // AFAIL = 1 discards ABOVE it
+        assert!(passes_alpha_test(0x40, 0x40, true));
+        assert!(
+            passes_alpha_test(0x20, 0x40, true),
+            "below passes when inverted"
+        );
+        assert!(
+            !passes_alpha_test(0x60, 0x40, true),
+            "above fails when inverted"
+        );
+    }
+
+    #[test]
+    fn the_opaque_bit_does_not_satisfy_the_alpha_test() {
+        // 0x80 is opaque, but it must not compare as 128 against a 0x40
+        // reference, or everything would pass.
+        assert!(
+            !passes_alpha_test(ALPHA_ONE, 0x40, true),
+            "the opaque bit leaked into the comparison"
+        );
+    }
+
+    #[test]
+    fn blend_modes_match_their_names() {
+        assert_eq!(BlendMode::from_primitive_type(0), BlendMode::Off);
+        assert_eq!(BlendMode::from_primitive_type(1), BlendMode::Source);
+        assert_eq!(BlendMode::from_primitive_type(2), BlendMode::Destination);
+        assert_eq!(
+            BlendMode::from_primitive_type(3),
+            BlendMode::SourcePlusDestination
+        );
+        assert_eq!(
+            BlendMode::from_primitive_type(200),
+            BlendMode::Off,
+            "out-of-range is Off"
+        );
+        assert_eq!(BlendMode::Off.blend(10, 250), 10);
+        assert_eq!(BlendMode::Destination.blend(10, 250), 250);
+        assert_eq!(
+            BlendMode::SourcePlusDestination.blend(200, 200),
+            255,
+            "clamps at the top"
+        );
+    }
+
     #[test]
     fn clamp_keeps_the_far_edge() {
         assert_eq!(clamp01(1.0), 1.0, "1.0 is the edge, not a wrap to 0");