Fix legend ordering in br_show_survival_curves to show groups in logical order

[Copilot]

The legend groups in br_show_survival_curves were displaying in alphabetical order instead of the intended logical order. This caused confusing displays where:

• Chinese labels: "中风险" (Medium Risk) appeared before "低风险" (Low Risk)
• Q labels: "Q10", "Q11" appeared before "Q2", "Q3"
• Default labels: "High Risk" appeared before "Low Risk" and "Medium Risk"

Root Cause

The issue was in line 658 of R/04-show.R where factor levels were converted to character strings:

plot_data$group <- gsub(".*=", "", plot_data$group)

This lost the intended factor level ordering, causing ggplot2 to sort legend items alphabetically.

Solution

After cleaning group names, the fix preserves the original factor level order:

# Clean group names while preserving factor level order
plot_data$group <- gsub(".*=", "", plot_data$group)

# Convert back to factor with the same level order as the original group_labels
# to ensure correct legend ordering
plot_data$group <- factor(plot_data$group, levels = group_labels)

Results

Now legends display in the correct logical order:

• 3 groups: "Low Risk" → "Medium Risk" → "High Risk"
• 5+ groups: "Q1" → "Q2" → "Q3" → "Q4" → "Q5"
• Chinese labels: "低风险" → "中风险" → "高风险"

Testing

Added comprehensive test coverage in test-survival-legend-order.R that validates:

• Correct factor level ordering for default 3-group scenarios
• Proper Q-label ordering for 5+ group scenarios
• All existing survival functionality continues to work

Fixes #47.

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