feat: add imgsecret embed/extract with DCT and majority voting

Implements DCT-based steganography module that hides 256-bit secrets in
JPEG luminance channel using Quantization Index Modulation (QIM) with
redundant copies and majority voting for reliable extraction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/lib.rs

@@ -9,3 +9,5 @@ pub use entry::{generate_entry_id, Entry, Manifest, ManifestEntry};
 
 pub mod vault;
 pub use vault::{decrypt_entry, decrypt_manifest, encrypt_entry, encrypt_manifest};
+
+pub mod imgsecret;