perf(qt): draw the spectrum in one ShaderEffect per mode (macOS F3)

The 28/48/5 Rectangle delegates cost one dirty node + batch churn each
per frame on Qt's Metal path (QRhiMetal::setShaderResources per batch
is where the ~6x vs Linux lives). Each mode's Loader now instantiates
ONE ShaderEffect — one node, one draw call: heights go to scalar
uniforms (b0..bN) pushed in one JS pass per settle tick, bar/gap/
gradient geometry is computed in the fragment shader with the same
colors and 1px rounding as the Rectangle version. The pre-existing
Repeater delegates are kept verbatim as the fallback arm (software
renderer or shader-compile error — the renderer preference added in
#104 makes that arm user-reachable).

Runtime findings on Qt 6.11 (documented in the file header): inline
GLSL strings are dead (fragmentShader is a URL property), so shaders
are baked with qsb --glsl 440 --msl 12 and shipped under
qml/assets/shaders/ (auto-qrc); and the built-in default vertex stage
emits no qt_TexCoord0, so one explicit shared vertex shader is baked
alongside.

Verified: build + 204/204 tests + both QML audits; RFB (fallback arm,
all three modes live during playback); Xvfb/GL (shader arm, all three
modes, zero shader errors; QSG_VISUALIZE=batches shows one uniform
batch block for the strip).

km

crates/qbz-qt/qml/assets/shaders/spectrum.vert

@@ -0,0 +1,17 @@
+#version 440
+
+    layout(location = 0) in vec4 qt_Vertex;
+    layout(location = 1) in vec2 qt_MultiTexCoord0;
+    layout(location = 0) out vec2 qt_TexCoord0;
+
+    layout(std140, binding = 0) uniform buf {
+        mat4 qt_Matrix;
+        float qt_Opacity;
+    };
+
+    out gl_PerVertex { vec4 gl_Position; };
+
+    void main() {
+        qt_TexCoord0 = qt_MultiTexCoord0;
+        gl_Position = qt_Matrix * qt_Vertex;
+    }

crates/qbz-qt/qml/assets/shaders/spectrum_bars.frag

@@ -0,0 +1,70 @@
+#version 440
+
+    layout(location = 0) in vec2 qt_TexCoord0;
+    layout(location = 0) out vec4 fragColor;
+
+    layout(std140, binding = 0) uniform buf {
+        mat4 qt_Matrix;
+        vec4 topColor;
+        vec4 bottomColor;
+        float qt_Opacity;
+        float bandW;
+        float bandH;
+        float b0;
+        float b1;
+        float b2;
+        float b3;
+        float b4;
+        float b5;
+        float b6;
+        float b7;
+        float b8;
+        float b9;
+        float b10;
+        float b11;
+        float b12;
+        float b13;
+    };
+
+    float amp(int i) {
+        if (i == 0) return b0;
+        if (i == 1) return b1;
+        if (i == 2) return b2;
+        if (i == 3) return b3;
+        if (i == 4) return b4;
+        if (i == 5) return b5;
+        if (i == 6) return b6;
+        if (i == 7) return b7;
+        if (i == 8) return b8;
+        if (i == 9) return b9;
+        if (i == 10) return b10;
+        if (i == 11) return b11;
+        if (i == 12) return b12;
+        return b13;
+    }
+
+    float rrectAlpha(vec2 p, float w, float h, float r) {
+        float rr = min(r, 0.5 * min(w, h));
+        vec2 q = abs(p - 0.5 * vec2(w, h)) - 0.5 * vec2(w, h) + rr;
+        float d = length(max(q, vec2(0.0))) + min(max(q.x, q.y), 0.0) - rr;
+        return 1.0 - smoothstep(-0.5, 0.5, d);
+    }
+
+    void main() {
+        vec2 px = qt_TexCoord0 * vec2(bandW, bandH);
+        float slot = bandW / 28.0;
+        int col = int(floor(px.x / slot));
+        col = clamp(col, 0, 27);
+        int m = col < 14 ? col : 27 - col;
+        float h = max(2.0, amp(m) * bandH);
+        float w = slot - 2.0;
+        vec2 lp = vec2(px.x - (float(col) * slot + 1.0), px.y - (bandH - h));
+        float shape = rrectAlpha(lp, w, h, 1.0);
+        if (shape <= 0.0) {
+            fragColor = vec4(0.0);
+            return;
+        }
+        vec4 g = mix(topColor, bottomColor, clamp(lp.y / h, 0.0, 1.0));
+        float a = g.a * shape * qt_Opacity;
+        fragColor = vec4(g.rgb * a, a);
+    }

crates/qbz-qt/qml/assets/shaders/spectrum_energy.frag

@@ -0,0 +1,51 @@
+#version 440
+
+    layout(location = 0) in vec2 qt_TexCoord0;
+    layout(location = 0) out vec4 fragColor;
+
+    layout(std140, binding = 0) uniform buf {
+        mat4 qt_Matrix;
+        vec4 topColor;
+        vec4 bottomColor;
+        float qt_Opacity;
+        float bandW;
+        float bandH;
+        float b0;
+        float b1;
+        float b2;
+        float b3;
+        float b4;
+    };
+
+    float amp(int i) {
+        if (i == 0) return b0;
+        if (i == 1) return b1;
+        if (i == 2) return b2;
+        if (i == 3) return b3;
+        return b4;
+    }
+
+    float rrectAlpha(vec2 p, float w, float h, float r) {
+        float rr = min(r, 0.5 * min(w, h));
+        vec2 q = abs(p - 0.5 * vec2(w, h)) - 0.5 * vec2(w, h) + rr;
+        float d = length(max(q, vec2(0.0))) + min(max(q.x, q.y), 0.0) - rr;
+        return 1.0 - smoothstep(-0.5, 0.5, d);
+    }
+
+    void main() {
+        vec2 px = qt_TexCoord0 * vec2(bandW, bandH);
+        float slot = bandW / 5.0;
+        int col = int(floor(px.x / slot));
+        col = clamp(col, 0, 4);
+        float h = max(2.0, amp(col) * bandH);
+        float w = slot - 6.0;
+        vec2 lp = vec2(px.x - (float(col) * slot + 3.0), px.y - (bandH - h));
+        float shape = rrectAlpha(lp, w, h, 1.0);
+        if (shape <= 0.0) {
+            fragColor = vec4(0.0);
+            return;
+        }
+        vec4 g = mix(topColor, bottomColor, clamp(lp.y / h, 0.0, 1.0));
+        float a = g.a * shape * qt_Opacity;
+        fragColor = vec4(g.rgb * a, a);
+    }

crates/qbz-qt/qml/assets/shaders/spectrum_wave.frag

@@ -0,0 +1,135 @@
+#version 440
+
+    layout(location = 0) in vec2 qt_TexCoord0;
+    layout(location = 0) out vec4 fragColor;
+
+    layout(std140, binding = 0) uniform buf {
+        mat4 qt_Matrix;
+        vec4 topColor;
+        float qt_Opacity;
+        float bandW;
+        float bandH;
+        float b0;
+        float b1;
+        float b2;
+        float b3;
+        float b4;
+        float b5;
+        float b6;
+        float b7;
+        float b8;
+        float b9;
+        float b10;
+        float b11;
+        float b12;
+        float b13;
+        float b14;
+        float b15;
+        float b16;
+        float b17;
+        float b18;
+        float b19;
+        float b20;
+        float b21;
+        float b22;
+        float b23;
+        float b24;
+        float b25;
+        float b26;
+        float b27;
+        float b28;
+        float b29;
+        float b30;
+        float b31;
+        float b32;
+        float b33;
+        float b34;
+        float b35;
+        float b36;
+        float b37;
+        float b38;
+        float b39;
+        float b40;
+        float b41;
+        float b42;
+        float b43;
+        float b44;
+        float b45;
+        float b46;
+        float b47;
+    };
+
+    float amp(int i) {
+        if (i == 0) return b0;
+        if (i == 1) return b1;
+        if (i == 2) return b2;
+        if (i == 3) return b3;
+        if (i == 4) return b4;
+        if (i == 5) return b5;
+        if (i == 6) return b6;
+        if (i == 7) return b7;
+        if (i == 8) return b8;
+        if (i == 9) return b9;
+        if (i == 10) return b10;
+        if (i == 11) return b11;
+        if (i == 12) return b12;
+        if (i == 13) return b13;
+        if (i == 14) return b14;
+        if (i == 15) return b15;
+        if (i == 16) return b16;
+        if (i == 17) return b17;
+        if (i == 18) return b18;
+        if (i == 19) return b19;
+        if (i == 20) return b20;
+        if (i == 21) return b21;
+        if (i == 22) return b22;
+        if (i == 23) return b23;
+        if (i == 24) return b24;
+        if (i == 25) return b25;
+        if (i == 26) return b26;
+        if (i == 27) return b27;
+        if (i == 28) return b28;
+        if (i == 29) return b29;
+        if (i == 30) return b30;
+        if (i == 31) return b31;
+        if (i == 32) return b32;
+        if (i == 33) return b33;
+        if (i == 34) return b34;
+        if (i == 35) return b35;
+        if (i == 36) return b36;
+        if (i == 37) return b37;
+        if (i == 38) return b38;
+        if (i == 39) return b39;
+        if (i == 40) return b40;
+        if (i == 41) return b41;
+        if (i == 42) return b42;
+        if (i == 43) return b43;
+        if (i == 44) return b44;
+        if (i == 45) return b45;
+        if (i == 46) return b46;
+        return b47;
+    }
+
+    float rrectAlpha(vec2 p, float w, float h, float r) {
+        float rr = min(r, 0.5 * min(w, h));
+        vec2 q = abs(p - 0.5 * vec2(w, h)) - 0.5 * vec2(w, h) + rr;
+        float d = length(max(q, vec2(0.0))) + min(max(q.x, q.y), 0.0) - rr;
+        return 1.0 - smoothstep(-0.5, 0.5, d);
+    }
+
+    void main() {
+        vec2 px = qt_TexCoord0 * vec2(bandW, bandH);
+        float slot = bandW / 48.0;
+        int col = int(floor(px.x / slot));
+        col = clamp(col, 0, 47);
+        float h = max(2.0, amp(col) * bandH);
+        float w = slot - 2.0;
+        vec2 lp = vec2(px.x - (float(col) * slot + 1.0), px.y - (bandH - h) * 0.5);
+        float shape = rrectAlpha(lp, w, h, 1.0);
+        if (shape <= 0.0) {
+            fragColor = vec4(0.0);
+            return;
+        }
+        float a = topColor.a * shape * qt_Opacity;
+        fragColor = vec4(topColor.rgb * a, a);
+    }