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/binary.rs

@@ -390,10 +390,12 @@ fn include_dsc_artifacts(
     checksums: &mut FileChecksums,
 ) -> Result<(), Box<dyn Error>> {
     let dsc_content = ctx.read_file(&upload_dir.join(dsc_name))?;
-    let para = crate::debian::control::parse_paragraphs(&dsc_content)
-        .into_iter()
-        .next()
-        .ok_or_else(|| format!("'{dsc_name}' is empty"))?;
+    let para = crate::debian::control::parse_paragraphs(
+        crate::debian::control::strip_clearsigned_armour(&dsc_content),
+    )
+    .into_iter()
+    .next()
+    .ok_or_else(|| format!("'{dsc_name}' is empty"))?;
 
     // Names and partial checksums are filled from the very same validated
     // lines, so a listed name can never miss its checksum entry.