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

README.md

@@ -5,7 +5,7 @@ and a configurable display.
 
 ## Features
 
-- **Three display modes** (top-right `MODE` knob):
+- **Three display modes** (`MODE` knob):
   - **BARS** — classic log-spaced spectrum bars with peak-hold caps.
   - **SPECTRUM** — a smooth (quadratic-spline) waveform trace of the spectrum,
     with a matching smooth peak-hold wave. Use the `SMOOTH` knob to go from
@@ -30,6 +30,10 @@ and a configurable display.
 | LEDS        | 4 – 48           | LEDs per bar in LED mode                      |
 | SMOOTH      | 0 – 1            | Waveform smoothing amount in SPECTRUM mode    |
 | GRID        | 0 – 1            | Grid (frequency/level) visibility / brightness|
+| ZOOM        | 98.0% – 102.0%   | Grid zoom about the plot centre               |
+| GRID FADE   | 0 – 1            | Fades inner grid lines; outer border stays   |
+| CURVING     | 0 – 1            | LED corner radius (0 = square, 1 = rounded)  |
+| TILT        | -1 – 1           | Spectral tilt: + boosts highs / cuts lows, - does the reverse (0 = flat) |
 
 ## Building