spec: add contour-filled specification

plots/contour-filled/specification.md

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+# contour-filled: Filled Contour Plot
+
+## Description
+
+A filled contour plot displays colored regions between level curves of a 2D scalar field, creating a smooth gradient visualization of continuous data. Unlike basic contour plots that show only isolines, filled contours use color bands to represent value ranges, making it easier to perceive gradual changes and identify regions of similar magnitude. This visualization is particularly effective for showing how a quantity varies continuously across a 2D surface.
+
+## Applications
+
+- Creating topographic maps with elevation bands showing terrain height ranges
+- Visualizing weather data such as temperature gradients, atmospheric pressure fields, or precipitation intensity
+- Displaying probability density estimates and statistical distributions in 2D space
+- Rendering mathematical functions to understand their behavior across a domain
+
+## Data
+
+- `x` (numeric) - X-axis coordinates forming a regular grid
+- `y` (numeric) - Y-axis coordinates forming a regular grid
+- `z` (numeric) - Scalar values at each (x, y) grid point representing the surface
+- Size: Grid of 30x30 to 100x100 points for smooth color transitions
+- Example: Mathematical function z = f(x, y) evaluated on a meshgrid, such as Gaussian peaks or saddle surfaces
+
+## Notes
+
+- Use a sequential or diverging colormap appropriate for the data range
+- Include a colorbar to indicate the value mapping
+- Consider overlaying contour lines for precise level identification
+- Adjust the number of levels (typically 10-20) for appropriate detail
+- Ensure grid resolution is sufficient to avoid jagged color boundaries

plots/contour-filled/specification.yaml

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+# Specification-level metadata for contour-filled
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: contour-filled
+title: Filled Contour Plot
+
+# Specification tracking
+created: 2025-12-29T23:55:20Z
+updated: null
+issue: 2500
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/concepts/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - contour
+    - heatmap
+  data_type:
+    - numeric
+    - continuous
+    - spatial
+  domain:
+    - general
+    - science
+    - engineering
+  features:
+    - filled
+    - gradient
+    - 2d