Add TILT spectral-tilt knob plus grid zoom/fade and curving controls

- TILT knob (-1..1, 0=flat) boosts highs / cuts lows about the log-frequency
  centre so high frequencies become more visible in the display
- Add GRID ZOOM, GRID FADE, and CURVING knobs; refine BARS/LEDS snapping
- Fix README MODE knob placement description

Source/Analyser.h

@@ -31,6 +31,8 @@ public:
     void setFalloffRate  (double rate)    noexcept { falloffRate = rate; }
     void setBarReleaseTau (double seconds) noexcept { barReleaseTau = seconds; }
     void setNumBands (int n) noexcept;
+    void setTilt (double amount) noexcept { tilt = amount; }
+    double getTilt() const noexcept { return tilt; }
     double getGraceSeconds() const noexcept { return grace; }
     double getFalloffRate()  const noexcept { return falloffRate; }
 
@@ -71,4 +73,10 @@ private:
     double barReleaseTau = 0.2; // seconds for the live bar to fall (release)
     double minDb = -48.0;      // dBFS-equivalent floor (bands below this -> 0)
     double maxDb = 0.0;        // dBFS-equivalent ceiling (bands at/above -> 1)
+
+    // Spectral tilt: +1 fully boosts the highs and cuts the lows, -1 does the
+    // opposite, 0 is flat. The gain is applied symmetrically about the band at
+    // the centre of the log-frequency range.
+    static constexpr double maxTiltDb = 18.0;
+    double tilt = 0.0;
 };