debian: fix deb822 writer/parser asymmetries

write_paragraph emitted a bare-space continuation line for empty lines
inside a value, which parse_paragraphs treated as a paragraph separator
and silently dropped the rest of the value; blank lines are now encoded
as ' .' like dpkg does and decoded back on read. Tab-indented
continuation lines now strip exactly one tab instead of keeping it.

Clearsigned .dsc content no longer leaks armor metadata into parsed
fields: the Hash:/Comment: header and the signature trailer are
stripped before parse_paragraphs at both .dsc parse sites.

src/build/mod.rs

@@ -376,10 +376,12 @@ pub fn run_source_build(
     // order the .dsc itself lists them.
     let dsc_content = std::fs::read_to_string(&ref_dsc_path)
         .map_err(|e| format!("cannot read '{}': {}", ref_dsc_path.display(), e))?;
-    let dsc_para = parse_paragraphs(&dsc_content)
-        .into_iter()
-        .next()
-        .ok_or_else(|| format!("'{}' is empty", ref_dsc_path.display()))?;
+    let dsc_para = parse_paragraphs(crate::debian::control::strip_clearsigned_armour(
+        &dsc_content,
+    ))
+    .into_iter()
+    .next()
+    .ok_or_else(|| format!("'{}' is empty", ref_dsc_path.display()))?;
 
     let mut tarball_paths = Vec::new();
     let mut dsc_file_names: Vec<String> = Vec::new();