docs(spectral_editor): Add feature roadmap for planned enhancements

Documented 6 planned features:
A. Shift+drag curve translation (2-3h)
B. Mouse wheel zoom/pan (6-8h)
C. Enhanced sinusoid patterns with asymmetric decay & modulation (8-12h)
D. Per-control-point parameter modulation (10-15h)
E. Composable profiles (Gaussian × Sinusoid) (12-16h)
F. Improved parameter slider ranges (3-4h)

Total estimated effort: 41-58 hours (1-1.5 weeks focused work)

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+# Spectral Editor - Feature Roadmap
+
+This document tracks planned enhancements for the spectral editor.
+
+## Priority: High
+
+### A. Curve Translation (Shift+Click+Drag)
+**Description**: Shift+click on a control point + mouse-move should displace the whole curve at a time (translate all control points).
+
+**Implementation Notes**:
+- Detect shift key state during control point click
+- Store initial positions of all control points in the curve
+- Apply uniform translation delta to all points during drag
+- Maintain curve shape while moving
+
+**Complexity**: Medium
+**Estimated Effort**: 2-3 hours
+
+---
+
+### B. Viewport Zoom (Mouse Wheel)
+**Description**: Mouse-wheel should allow zooming in/out on the view for fine placement of curves.
+
+**Implementation Notes**:
+- Implement zoom scale factor (e.g., 0.5x to 4.0x)
+- Center zoom around mouse cursor position
+- Update rendering to use scaled coordinates
+- Add visual zoom indicator (e.g., "Zoom: 2.0x")
+- Consider pan functionality (drag with middle mouse or space+drag)
+
+**Complexity**: High (coordinate transformation, pan interaction)
+**Estimated Effort**: 6-8 hours
+
+---
+
+## Priority: Medium
+
+### C. Enhanced Sinusoid Pattern
+**Description**: The 'sinusoid' pattern is quite interesting and should have more variations.
+
+**Proposed Variations**:
+- **Asymmetric Decay**: Different decay rates above and below the curve center
+  - `decay_top` parameter (controls upper harmonics falloff)
+  - `decay_bottom` parameter (controls lower harmonics falloff)
+- **Temporal Modulation**: Per-frame amplitude/frequency modulation along timeline
+  - `amplitude_envelope` (fade in/out over time)
+  - `frequency_drift` (vibrato/wobble effect)
+  - `phase_offset` (shift pattern over time)
+- **Harmonic Series**: Option to generate harmonic overtones
+  - `num_harmonics` parameter
+  - `harmonic_decay` parameter
+
+**Implementation Notes**:
+- Extend `SinusoidProfile` class with additional parameters
+- Add UI controls for new parameters (sliders, dropdowns)
+- Render preview showing modulation over time
+
+**Complexity**: Medium-High
+**Estimated Effort**: 8-12 hours
+
+---
+
+### D. Per-Control-Point Modulation
+**Description**: Each control point should be assigned individually controllable volume, decay params, etc. for fine modulation along time.
+
+**Proposed Parameters (per control point)**:
+- `volume`: Local amplitude multiplier (0.0 - 2.0)
+- `decay`: Local decay rate override
+- `width`: Gaussian width override (for profile spreading)
+- `phase`: Phase offset for sinusoid patterns
+- `color`: Visual indicator for parameter variations
+
+**Implementation Notes**:
+- Extend control point data structure with parameter fields
+- Add per-point property panel (show on control point selection)
+- Render visual hints (color-coded points, size variations)
+- Interpolate parameters between control points for smooth transitions
+
+**Complexity**: High (UI/UX design, parameter interpolation)
+**Estimated Effort**: 10-15 hours
+
+---
+
+### E. Composable Profiles
+**Description**: Profiles should be composable along a curve (e.g., apply Gaussian curve to sinusoid pattern).
+
+**Proposed Syntax**:
+```cpp
+// Example: Gaussian-modulated sinusoid
+CompositeProfile {
+  base: SinusoidProfile { frequency: 100.0, decay: 0.5 },
+  envelope: GaussianProfile { center: 256, width: 50 }
+}
+```
+
+**Implementation Notes**:
+- Define profile composition operators:
+  - `multiply`: Envelope modulation (amplitude × profile)
+  - `add`: Additive blending (profile1 + profile2)
+  - `max`: Take maximum value at each bin
+- Add UI for profile layering (drag-and-drop profile stack)
+- Render composite preview with layer visualization
+
+**Complexity**: High (requires profile abstraction refactor)
+**Estimated Effort**: 12-16 hours
+
+---
+
+## Priority: Low (Polish)
+
+### F. Improved Parameter Sliders
+**Description**: Adjust slider ranges for better usability (Decay, Width, Frequency, etc.).
+
+**Issues to Address**:
+- Decay slider: Non-linear scaling (logarithmic?) for finer control at low values
+- Frequency slider: Snap to musical notes (optional A440-based grid)
+- Width slider: Preview visualization (show affected frequency range)
+- General: Add numeric input fields next to sliders for precise values
+
+**Implementation Notes**:
+- Implement logarithmic slider interpolation for decay/width
+- Add slider tick marks at useful intervals
+- Display current value and units (Hz, bins, dB, etc.)
+- Add reset-to-default buttons
+
+**Complexity**: Low-Medium
+**Estimated Effort**: 3-4 hours
+
+---
+
+## Future Ideas (Backlog)
+
+- **Undo/Redo System**: Track edit history for curve modifications
+- **Preset Library**: Save/load common curve patterns (kick drum, snare, bass, etc.)
+- **Curve Smoothing**: Apply smoothing filters to jittery control points
+- **Copy/Paste**: Duplicate curves or control point selections
+- **Multi-Selection**: Select and edit multiple control points simultaneously
+- **Grid Snapping**: Snap control points to frequency/time grid
+- **Export Options**: Export to different formats (JSON, binary, C++ code)
+
+---
+
+## Total Estimated Effort
+- **High Priority**: 8-11 hours
+- **Medium Priority**: 30-43 hours
+- **Low Priority**: 3-4 hours
+- **Grand Total**: 41-58 hours (roughly 1-1.5 weeks of focused work)
+
+---
+
+*Last Updated: February 6, 2026*