feat(letsplot): implement ridgeline-basic (#548)

## Summary
Implements `ridgeline-basic` for **letsplot**.

**Parent Issue:** #539
**Base Branch:** `plot/ridgeline-basic`

## Files
- `plots/ridgeline-basic/implementations/letsplot.py`

## Preview
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Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: claude[bot]

plots/ridgeline-basic/implementations/letsplot.py

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+"""
+ridgeline-basic: Ridgeline Plot
+Library: letsplot
+"""
+
+import os
+import shutil
+
+import numpy as np
+import pandas as pd
+from lets_plot import *
+
+
+LetsPlot.setup_html()
+
+# Data - monthly temperature distributions
+np.random.seed(42)
+months = [
+    "January",
+    "February",
+    "March",
+    "April",
+    "May",
+    "June",
+    "July",
+    "August",
+    "September",
+    "October",
+    "November",
+    "December",
+]
+
+# Generate realistic temperature data with seasonal pattern
+data = []
+for i, month in enumerate(months):
+    # Seasonal temperature pattern (Northern Hemisphere)
+    base_temp = 5 + 15 * np.sin((i - 3) * np.pi / 6)
+    temps = np.random.normal(base_temp, 4, 200)
+    for temp in temps:
+        data.append({"month": month, "temperature": temp})
+
+df = pd.DataFrame(data)
+
+# Create ordered categorical for proper month ordering
+df["month"] = pd.Categorical(df["month"], categories=months, ordered=True)
+
+# Color palette gradient from cool to warm
+colors = [
+    "#306998",  # Python Blue (winter)
+    "#3B82F6",  # Blue
+    "#06B6D4",  # Cyan
+    "#10B981",  # Emerald
+    "#22C55E",  # Green
+    "#84CC16",  # Lime
+    "#FFD43B",  # Python Yellow (summer)
+    "#F97316",  # Orange
+    "#EF4444",  # Red
+    "#F97316",  # Orange
+    "#8B5CF6",  # Violet
+    "#306998",  # Python Blue
+]
+
+# Create ridgeline plot using geom_area_ridges
+plot = (
+    ggplot(df, aes(x="temperature", y="month", fill="month"))
+    + geom_area_ridges(alpha=0.7, scale=1.5, color="white", size=0.5)
+    + scale_fill_manual(values=colors)
+    + labs(x="Temperature (°C)", y="Month", title="Monthly Temperature Distribution")
+    + theme_minimal()
+    + theme(
+        axis_text=element_text(size=16),
+        axis_title=element_text(size=20),
+        plot_title=element_text(size=20),
+        legend_position="none",
+    )
+    + ggsize(1600, 900)
+)
+
+# Save as PNG and HTML
+# Note: lets_plot saves to lets-plot-images/ subdirectory by default
+ggsave(plot, "plot.png", scale=3)
+ggsave(plot, "plot.html")
+
+# Move files from lets-plot-images to current directory
+if os.path.exists("lets-plot-images/plot.png"):
+    shutil.move("lets-plot-images/plot.png", "plot.png")
+if os.path.exists("lets-plot-images/plot.html"):
+    shutil.move("lets-plot-images/plot.html", "plot.html")
+if os.path.exists("lets-plot-images") and not os.listdir("lets-plot-images"):
+    os.rmdir("lets-plot-images")