1.1.1: AACS decrypt + key-resolution hardening

- decrypt_unit: padding-aware acceptance — recover real video at content-
  fragment tails (the phantom mux-loss class) without weakening wrong-key
  rejection (a full content unit still needs all 32 TS syncs).
- scan: read the MKB via the bounded read_mkb_content so Disc::inputs()
  carries it. Online key resolution was shipping mkb=0 (a full read of the
  ~128 MiB MKB_RO allocation fails) → the decode service 404'd.
- resolve_vid_only: surface an MKB read error instead of silently emptying.
- fetch: a per-sample dry-set replaces the global fetch_spent latch, so a
  second CPS unit's key can still be fetched after the first came back empty.
- verify::push_ranges: saturating arithmetic (corrupt-disc panic guard).
- Tests for all of the above.

src/disc/verify.rs

@@ -477,7 +477,9 @@ fn push_ranges(out: &mut Vec<(u32, u32)>, lbas: &[u32; 3]) {
     present.sort_unstable();
     for lba in present {
         if let Some(last) = out.last_mut() {
-            if last.0 + last.1 == lba {
+            // Saturating: LBAs come from disc-controlled ICB extents, so a
+            // corrupt disc must not panic here (matches `udf::merge_ranges`).
+            if last.0.saturating_add(last.1) == lba {
                 last.1 += 1;
                 continue;
             }