Clean up for public release: docs, zero warnings, no hardcoded paths

Documentation:
- docs/aacs.md — AACS encryption (1.0 + 2.0), key resolution, decrypt
- docs/udf.md — UDF 2.50 filesystem with metadata partitions
- docs/mpls.md — MPLS playlist format, STN stream table
- docs/clpi.md — CLPI clip info, EP map, sector extents
- docs/architecture.md — library module map, design principles
- docs/drive-access.md — drive sessions, SCSI transport, unlock

Code cleanup:
- Zero compiler warnings
- Removed all debug eprintln from library code
- No hardcoded private paths — KEYDB tests use KEYDB_PATH env var
- KEYDB search locations as named constants
- drive.rs: extracted create_platform(), deduplicated open methods
- lib.rs: updated doc examples to show Disc::scan() API
- Fixed UDF file reads (partition_start, not metadata_start)
- Exported KeySource from disc module

src/udf.rs

@@ -221,7 +221,7 @@ pub fn read_filesystem(session: &mut DriveSession) -> Result<UdfFs> {
 
         // Parse partition maps starting at offset 440
         // Map 0 = Type 1 (physical), Map 1 = Type 2 (metadata)
-        let pm1_type = lvd[440]; // First map type
+        let _pm1_type = lvd[440]; // First map type
         let pm1_len = lvd[441] as usize;
 
         if pm1_len > 0 && 440 + pm1_len < 2048 {