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/debian/mod.rs

@@ -24,6 +24,8 @@ pub use changelog::{
     parse_previous_version_from_str,
 };
 pub use checksums::{Entry as ChecksumEntry, FileChecksums};
-pub use control::{ControlInfo, Paragraph, parse_paragraphs, write_paragraph};
+pub use control::{
+    ControlInfo, Paragraph, parse_paragraphs, strip_clearsigned_armour, write_paragraph,
+};
 pub use files::{FilesEntry, FilesList};
 pub use version::DebianVersion;