FMTS: resolve the index key map from one forensic keyserver query

The keyserver protocol now returns all 32 index keys as an array for a
forensic content sample (and a single-element array for plain content).
resolve_fmts_key_map sends one forensic batch and maps array element i to
segment index i+1, replacing the per-index blind-probe collection loop
that repeatedly hit the key service. Segment/index parsing and the
aligned-unit content classification are reworked to support this:

- rename variant_select -> index_select (per-index, not per-variant)
- content classification moves to is_clean(buf, ContentFormat) so the
  unit selector emits only units the key service accepts
- segment.rs: parse IndividualSegment.tbl index tags + SPN ranges,
  build contiguous LBA key ranges from the resolved 32-key array
- decrypt/decorator plumbing for the resolved per-index keys

Fail loud (FmtsKeyMissing) when the forensic query returns < 32 keys or
any segment index stays unresolved.

src/aacs/types.rs

@@ -57,41 +57,40 @@ pub struct ProcessingKey(pub [u8; 16]);
 pub struct UnitKey {
     pub idx: u32,
     pub key: [u8; 16],
-    /// AACS 2.1 (FMTS) forensic-variant tag.
+    /// AACS 2.1 (FMTS) forensic **index** tag (see [`crate::aacs::segment`]).
     ///
     /// `0` = ordinary (non-forensic) content — the value for every 1.0 / 2.0
-    /// key and for the bulk of a 2.1 title. `1..=32` = a variant key that
-    /// decrypts the forensic segments tagged with that same variant in
-    /// `IndividualSegment.tbl`. A disc resolves to exactly one variant, so at
-    /// most one non-zero value is ever in play for a given rip; the decode
-    /// selects the segments matching it and drops the other variants.
-    pub variant_number: u8,
+    /// key and for the bulk of a 2.1 title. `1..=32` = a forensic index key that
+    /// decrypts the `IndividualSegment.tbl` segments tagged with that same index.
+    /// This is the per-segment index (1..32), NOT the AACS 2.1 Media Key Variant
+    /// (the 65536-value device selector), which is a separate MKB-layer concern.
+    pub index_number: u8,
 }
 
 impl UnitKey {
-    /// An ordinary (non-forensic) unit key: `variant_number == 0`. The value
+    /// An ordinary (non-forensic) unit key: `index_number == 0`. The value
     /// for every AACS 1.0 / 2.0 key and the bulk of a 2.1 title.
     pub const fn new(idx: u32, key: [u8; 16]) -> Self {
         Self {
             idx,
             key,
-            variant_number: 0,
+            index_number: 0,
         }
     }
 
-    /// A forensic-variant key: `variant_number` in `1..=32`, decrypting the
-    /// `IndividualSegment.tbl` segments tagged with that variant.
-    pub const fn variant(idx: u32, key: [u8; 16], variant_number: u8) -> Self {
+    /// A forensic index key: `index_number` in `1..=32`, decrypting the
+    /// `IndividualSegment.tbl` segments tagged with that index.
+    pub const fn forensic(idx: u32, key: [u8; 16], index_number: u8) -> Self {
         Self {
             idx,
             key,
-            variant_number,
+            index_number,
         }
     }
 
-    /// Whether this key decrypts ordinary (non-forensic) content.
-    pub const fn is_default_variant(&self) -> bool {
-        self.variant_number == 0
+    /// Whether this key decrypts ordinary (non-forensic) content (index 0).
+    pub const fn is_default_index(&self) -> bool {
+        self.index_number == 0
     }
 }