feat: add WGSL debug_f32() snippet with C64 8x8 font

Renders a f32 value (±999.999, 3 decimal digits) at a given screen
position using authentic C64 8x8 bitmap glyphs in yellow.
Usage: col = debug_f32(col, pos.xy, vec2f(10.0, 10.0), value);
Include: #include "debug/debug_print"

handoff(Gemini): new snippet at src/shaders/debug/debug_print.wgsl

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 67 insertions, 0 deletions

diff --git a/src/shaders/debug/debug_print.wgsl b/src/shaders/debug/debug_print.wgsl
new file mode 100644
index 0000000..ee9b107
--- /dev/null
+++ b/src/shaders/debug/debug_print.wgsl
@@ -0,0 +1,67 @@
+// Debug print utility: renders a f32 value at a pixel position using 8×8 C64 font.
+// Include: #include "debug/debug_print"
+//
+// Usage:
+//   col = debug_f32(col, pos.xy, vec2f(10.0, 10.0), my_value);
+//
+// Format: [-]DDD.DDD  (8 chars × 8px wide = 64×8 pixel area)
+// Range:  -999.999 to 999.999
+
+// Returns the lit pixel (0 or 1) for glyph g, pixel row r [0-7], column c [0-7].
+// Glyphs: 0-9 = digits, 10 = '-', 11 = '.', 12 = ' '
+// Encoding: 2 u32s per glyph; hi covers rows 0-3, lo covers rows 4-7.
+// Within each u32, row r occupies bits [(3-r%4)*8 + 7 : (3-r%4)*8].
+fn _dbg_pixel(g: u32, r: u32, c: u32) -> u32 {
+    let data = array<u32, 26>(
+        0x3C666E76u, 0x66663C00u,  // '0'
+        0x18381818u, 0x18187E00u,  // '1'
+        0x3C66060Cu, 0x18307E00u,  // '2'
+        0x3C66061Cu, 0x06663C00u,  // '3'
+        0x0C1C2C4Cu, 0x7E0C0C00u,  // '4'
+        0x7E607C06u, 0x06663C00u,  // '5'
+        0x1C30607Cu, 0x66663C00u,  // '6'
+        0x7E060C18u, 0x18181800u,  // '7'
+        0x3C66663Cu, 0x66663C00u,  // '8'
+        0x3C66663Eu, 0x060C3800u,  // '9'
+        0x0000007Eu, 0x00000000u,  // '-'
+        0x00000000u, 0x00181800u,  // '.'
+        0x00000000u, 0x00000000u,  // ' '
+    );
+    let word = data[g * 2u + (r / 4u)];
+    let shift = (3u - (r % 4u)) * 8u + (7u - c);
+    return (word >> shift) & 1u;
+}
+
+// Overlays printed value onto col, returning updated RGB.
+// pixel_pos : @builtin(position).xy
+// origin    : top-left corner of text in screen pixels
+// value     : f32 to display
+fn debug_f32(col: vec3f, pixel_pos: vec2f, origin: vec2f, value: f32) -> vec3f {
+    let ink_color = vec3f(1.0, 1.0, 0.0);  // yellow
+    let lp = pixel_pos - origin;
+    if (lp.x < 0.0 || lp.x >= 64.0 || lp.y < 0.0 || lp.y >= 8.0) {
+        return col;
+    }
+    let ix = u32(lp.x);
+    let iy = u32(lp.y);
+    let char_col = ix / 8u;
+    let bit_col  = ix % 8u;
+
+    let neg      = value < 0.0;
+    let abs_val  = min(abs(value), 999.999f);
+    let int_part = u32(abs_val);
+    let frac_s   = min(u32((abs_val - f32(int_part)) * 1000.0 + 0.5), 999u);
+
+    var g: u32;
+    switch char_col {
+        case 0u:    { g = select(12u, 10u, neg); }       // sign
+        case 1u:    { g = (int_part / 100u) % 10u; }     // hundreds
+        case 2u:    { g = (int_part / 10u)  % 10u; }     // tens
+        case 3u:    { g = int_part % 10u; }               // ones
+        case 4u:    { g = 11u; }                          // '.'
+        case 5u:    { g = (frac_s / 100u) % 10u; }       // tenths
+        case 6u:    { g = (frac_s / 10u)  % 10u; }       // hundredths
+        default:    { g = frac_s % 10u; }                 // thousandths
+    }
+    return mix(col, ink_color, f32(_dbg_pixel(g, iy, bit_col)));
+}