Fix read speed: detect kernel max transfer size, fix phantom streams

Root cause: READ_10 requested 510 sectors (1MB) but kernel sg driver
limits to max_hw_sectors_kb=120 (60 sectors). All bulk reads errored,
error handler fell to 3-sector reads → 4.8 MB/s instead of 12+ MB/s.

- detect_max_batch_sectors() now resolves sg→block device via sysfs
 and uses 80% of kernel limit for safety margin
- Default fallback reduced from 510 to 48 sectors
- ContentReader uses per-device detected max, not hardcoded constant
- Filter out coding_type 0x00 (empty/padding) stream entries
- Add MPLS stream_type 5/6/7 attribute parsing (secondary audio/video)

Tested on BU40N: 4.8 → 12.5 MB/s sustained, 23.7 MB/s peak.

src/mpls.rs

@@ -263,6 +263,27 @@ fn parse_stream_entry(item: &[u8], pos: usize, stream_type: u8) -> Option<(Strea
                 language = String::from_utf8_lossy(&sa[1..4]).to_string();
             }
         }
+        5 => {
+            // Secondary audio: same as primary audio
+            if sa.len() >= 2 {
+                audio_format = (sa[1] >> 4) & 0x0F;
+                audio_rate = sa[1] & 0x0F;
+            }
+            if sa.len() >= 5 {
+                language = String::from_utf8_lossy(&sa[2..5]).to_string();
+            }
+        }
+        6 | 7 => {
+            // Secondary video: same as primary video
+            if sa.len() >= 2 {
+                video_format = (sa[1] >> 4) & 0x0F;
+                video_rate = sa[1] & 0x0F;
+            }
+            if coding_type == 0x24 && sa.len() > 2 {
+                dynamic_range = (sa[2] >> 4) & 0x0F;
+                color_space_val = sa[2] & 0x0F;
+            }
+        }
         _ => {}
     }