test(disc): pin the -t 1 = main-feature contract (DVD); fix 2 test clippy nits

Owner-flagged invariant: freemkv -t 1 ALWAYS selects the main feature because the
CLI's title 1 maps to titles[0] and the list is ordered by canonical_title_order
(main feature first). Adds a DVD-shaped contract pin asserting titles[0] is the
movie after sorting (a regression there is a title-ordering bug, not a remux issue),
complementing the existing branching-UHD / normal-disc order tests.

Also clears two pre-existing test-only clippy nits surfaced under --all-targets:
unused `lba` in ClearStubReader::read_sectors, and an unneeded `mut` on the
h264 population test's closure.

src/mux/codec/h264.rs

@@ -617,7 +617,7 @@ mod tests {
         //   0x98 = '1 00110..' → slice_type 5 (P)
         //   0x9C = '1 00111..' → slice_type 6 (B)
         let src = crate::pes::SourcePos::at_byte(8192);
-        let mut parse = |nal_type: u8, body: u8| {
+        let parse = |nal_type: u8, body: u8| {
             let mut p = H264Parser::new();
             let mut pe = make_pes(h264_nal(nal_type, &[body]), Some(0));
             pe.source = Some(src);