adding more nodes

docs/nodes/Synthetic Surface.md

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+# Synthetic Surface
+
+Generate synthetic test surfaces for development, calibration, and algorithm testing. Equivalent to Gwyddion's *_synth.c modules.
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| surface | DATA_FIELD | Generated synthetic surface |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| pattern | dropdown | fbm | Pattern type: fbm, white_noise, lattice, steps, particles, flat |
+| xres | INT | 256 | Horizontal resolution in pixels (16–2048) |
+| yres | INT | 256 | Vertical resolution in pixels (16–2048) |
+| xreal | FLOAT | 1e-6 | Physical width in metres |
+| yreal | FLOAT | 1e-6 | Physical height in metres |
+| amplitude | FLOAT | 1e-9 | Peak-to-peak amplitude in metres |
+| seed | INT | 42 | Random seed for reproducibility (0–999999) |
+| hurst_exponent | FLOAT | 0.7 | FBM roughness exponent: 0 = rough, 1 = smooth (fbm only) |
+| lattice_spacing | FLOAT | 100e-9 | Lattice period in metres (lattice only) |
+| lattice_angle | FLOAT | 90.0 | Angle between lattice vectors in degrees (lattice only) |
+| n_steps | INT | 5 | Number of step terraces (steps only) |
+| n_particles | INT | 20 | Number of particles (particles only) |
+| particle_radius_px | INT | 10 | Particle radius in pixels (particles only) |
+
+## Notes
+
+- **fbm**: Fractional Brownian motion via spectral synthesis. Hurst exponent controls roughness.
+- **white_noise**: Gaussian random noise.
+- **lattice**: Two-axis sinusoidal grid with configurable spacing and angle.
+- **steps**: Terraced step structure with equal step heights.
+- **particles**: Random spherical particles on a flat background.
+- **flat**: Zero surface (useful as a baseline).
+- All patterns are normalised to the specified amplitude range.