fix: correct MTEXT line spacing and preserve blank paragraph separators

Line spacing: DXF code 44 (line_spacing_factor) is a multiplier on
AutoCAD's default baseline-to-baseline distance of 5/3 × text_height.
The renderer was using factor × 1.0 instead of factor × 1.667, making
all MTEXT appear cramped.

Blank paragraphs: split_mtext_lines was filtering out empty strings, so
\\P\\P (double paragraph break used to separate sections) was silently
collapsed. Empty lines in the middle of the block are now preserved;
only leading/trailing blank lines are stripped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/entities/text_support.rs

@@ -206,10 +206,14 @@ pub fn strip_mtext_codes(s: &str) -> String {
 }
 
 pub fn split_mtext_lines(s: &str) -> Vec<String> {
-    s.split('\n')
+    let lines: Vec<String> = s.split('\n')
         .map(|l| l.trim().to_string())
-        .filter(|l| !l.is_empty())
-        .collect()
+        .collect();
+    // Drop leading and trailing blank lines, but preserve blank lines in the
+    // middle — they are intentional paragraph separators (\\P\\P in MTEXT).
+    let start = lines.iter().position(|l| !l.is_empty()).unwrap_or(0);
+    let end = lines.iter().rposition(|l| !l.is_empty()).map(|i| i + 1).unwrap_or(0);
+    lines[start..end].to_vec()
 }
 
 /// Measure the advance width of an MText line (after strip_mtext_codes), correctly