feat(python): add VegaLiteWriter.render_chart() for Altair output

[cpsievert]

Summary

The two-stage API (documented in the README) separates reading from writing, giving users control over each step. However, the writing step currently only produces a raw JSON string — users who want an Altair chart object need to manually dispatch to the right Altair class (Chart, LayerChart, FacetChart, etc.) based on the spec structure.

This PR adds VegaLiteWriter.render_chart() so the two-stage API can produce Altair charts directly, just like render_altair() does for the one-step path:

writer = ggsql.VegaLiteWriter()

# Before: raw JSON only
json_str = writer.render(spec)

# Now: also get an Altair chart object
chart = writer.render_chart(spec)

Implementation details:

• Wraps the Rust VegaLiteWriter in a Python class that adds render_chart()
• Extracts the Altair chart-type dispatch logic from render_altair() into a shared _json_to_altair_chart() helper (deduplication, not new logic)
• No Rust changes required

Test plan

• 3 new tests in TestVegaLiteWriterRenderChart (basic, layered, faceted)
• Full test suite passes

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