feat(altair): implement scatter-basic (#500)

## Summary
Implements `scatter-basic` for **altair** library.

- Basic scatter plot with 100 random data points showing correlation
- Uses Python Blue (#306998) from style guide
- Configured for 4800x2700px output (1600x900 with scale_factor=3)
- Includes tooltips for interactivity
- Proper axis labels, title, and font sizing

**Parent Issue:** #207
**Sub-Issue:** #441
**Base Branch:** `plot/scatter-basic`
**Attempt:** 1/3

## Implementation
- `plots/altair/point/scatter-basic/default.py`

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>

Author: claude[bot]

plots/altair/point/scatter-basic/default.py

@@ -19,7 +19,7 @@
 chart = (
     alt.Chart(data)
     .mark_point(filled=True, opacity=0.7, size=100, color="#306998")
-    .encode(x=alt.X("x:Q", title="X Value"), y=alt.Y("y:Q", title="Y Value"))
+    .encode(x=alt.X("x:Q", title="X Value"), y=alt.Y("y:Q", title="Y Value"), tooltip=["x:Q", "y:Q"])
     .properties(width=1600, height=900, title="Basic Scatter Plot")
     .configure_axis(labelFontSize=16, titleFontSize=20)
     .configure_title(fontSize=20)