Add plot titles showing BAM filename and fix boxplot layout

[ewels]

Summary

• Add plot titles: Display the input BAM filename (without .bam suffix) as a bold title centered at the top of all three plot types (density scatter, boxplot, histogram)
• Fix boxplot x-axis layout: Reduce excessive x_label_area_size from 150→70, eliminating the large gap between rotated tick labels and the x-axis description label

Details

Title implementation

• Added title: &str parameter to all render functions and their public wrappers
• Canvas is split vertically into a 35px title area + the plot area
• Title is drawn in bold 18pt font, centered horizontally with 8px top margin
• Canvas heights increased to accommodate: scatter/histogram 480→520, boxplot 480→560
• bam_stem (already computed in main.rs) is passed through to all plot calls

Boxplot layout fix

• x_label_area_size: 150→70 (was creating a huge gap between axis and label)
• margin_bottom: 10→5

Verification

• Tested with small benchmark (test.bam) and large benchmark (GM12878_REP1.markdup.sorted.bam)
• All 24 unit tests + 8 integration tests pass
• cargo fmt --check and cargo clippy -- -D warnings clean