Round 4: fix 26 defects across crypto, resource use and codec paths

Twenty-six confirmed findings from the fourth audit round, landed as one
cluster because they were found by agents working over disjoint file sets.

The one worth calling out is a pair of AACS tests that could not fail.
Both asserted CBC behaviour against a hand-rolled expectation that
happened to be IV-independent, so replacing AACS_IV with sixteen zero
bytes left them passing — they were pinning the code's own arithmetic,
not the published constant. Replaced with a literal witness of the
published IV plus the NIST SP 800-38A F.2.2 CBC-AES128 vector, and
verified the other way round: zeroing AACS_IV now fails three tests.

The rest are allocation and correctness work on hot paths: the Annex-B
writer in demux_sink allocated and freed a whole-frame Vec per frame,
which for a UHD title is ~200,000 allocations over the mmap threshold
plus the page faults to first-touch each one; it now reuses a buffer on
the writer, and still takes the NAL prefix width from the configuration
record rather than assuming four.

Six findings whose real fix lives in a consumer crate are recorded for
re-filing rather than patched here.

src/io/writeback_file/macos.rs

@@ -112,8 +112,26 @@ pub(super) fn durable_sync(file: &File) -> io::Result<()> {
             );
             Ok(())
         }
-        Err(crate::io::bounded::BoundedError::Halted) => Ok(()),
-        Err(crate::io::bounded::BoundedError::WorkerLost) => Ok(()),
+        // Both arms below used to map to `Ok(())` with NO diagnostic at all, while
+        // the Linux sibling logs the identical failures (writeback_file/linux.rs).
+        // A lost F_FULLFSYNC worker at the end of a UHD mux therefore reported
+        // `completed = true` with an empty log, leaving an operator investigating a
+        // truncated/corrupt output file after a power loss no record that the final
+        // fsync never ran — on Linux the same failure is at error level.
+        Err(crate::io::bounded::BoundedError::Halted) => {
+            tracing::warn!(
+                target: "mux",
+                "WritebackFile::sync_all F_FULLFSYNC skipped (halt requested); data not durably flushed, kernel will flush on close"
+            );
+            Ok(())
+        }
+        Err(crate::io::bounded::BoundedError::WorkerLost) => {
+            tracing::error!(
+                target: "mux",
+                "WritebackFile::sync_all F_FULLFSYNC worker lost before completion; data not durably flushed, kernel will flush on close"
+            );
+            Ok(())
+        }
     }
 }