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/mux/videomap.rs

@@ -20,7 +20,7 @@
 //! different output format would be a DIFFERENT sink reusing this same model,
 //! not a pluggable encoder here.
 
-use crate::disc::{ColorSpace, DiscTitle, FrameRate, Stream as DiscStream, VideoStream};
+use crate::disc::{ColorSpace, DiscTitle, Stream as DiscStream, VideoStream};
 use crate::mux::codec::PictureInfo;
 use crate::mux::codec::coding::{CodingType, FieldOrder};
 use crate::pes::{PesFrame, SourcePos};
@@ -268,13 +268,6 @@ fn display_aspect_ratio(v: &VideoStream, w: u32, h: u32) -> (u32, u32) {
     }
 }
 
-/// The title's nominal frame rate as a fraction — the single mapping site reused
-/// by the header builder. (Retained as the canonical accessor.)
-#[allow(dead_code)]
-fn frame_rate_fraction(fr: FrameRate) -> (u32, u32) {
-    fr.as_fraction()
-}
-
 /// One per-picture index record, distilled from a video [`PesFrame`]
 /// (`docs/FVI_FORMAT.md` §7).
 ///