Add rtf_rich_text() function for inline text formatting

Summary

r2rtf provides rtf_rich_text() function for inline text formatting within table cells. This allows mixing different text styles (bold, italic, etc.) within a single cell.

Expected functionality

# r2rtf example
rtf_rich_text("Normal text {b Bold text} and {i italic text}")

Proposed Python equivalent

# Proposed rtflite implementation
rtf.rich_text("Normal text {b Bold text} and {i italic text}")

Minimal example

import rtflite as rtf
import polars as pl

df = pl.DataFrame({
    "Treatment": ["Placebo", "Drug A"],
    "Response": [
        rtf.rich_text("n=50 {b (75%)}"),
        rtf.rich_text("n=48 {b (92%)}")
    ]
})

doc = rtf.RTFDocument(df=df)
doc.write_rtf("rich_text_example.rtf")

Use case

Clinical reports often need mixed formatting within cells (e.g., "n=50 {b (75%)}" where the percentage is bold).

References

• r2rtf rtf_rich_text documentation

Start by tracing how RTFDocument consumes DataFrame cell values. Use the examples to define the supported {b ...} and {i ...} inline syntax, then verify that mixed text is emitted correctly in an RTF table cell, including the clinical-report example.