fix(core): correct off-by-one in imgsecret SOF bounds guard

peek_jpeg_dimensions reads jpeg[i+8] as the last byte, so the guard
should be \`i + 8 >= jpeg.len()\`, not \`i + 9 >= jpeg.len()\`. The old
guard would reject a valid SOF marker ending exactly at len()-1.
Caught in Task 2 code-quality review.

crates/relicario-core/src/imgsecret.rs

@@ -139,7 +139,7 @@ fn peek_jpeg_dimensions(jpeg: &[u8]) -> Result<(u32, u32)> {
             } // SOI / EOI
             0xC0..=0xC3 | 0xC5..=0xC7 | 0xC9..=0xCB | 0xCD..=0xCF => {
                 // SOFn — height in [i+5..i+7], width in [i+7..i+9]
-                if i + 9 >= jpeg.len() {
+                if i + 8 >= jpeg.len() {
                     return Err(RelicarioError::ImgSecret("truncated SOF marker".into()));
                 }
                 let height = u16::from_be_bytes([jpeg[i + 5], jpeg[i + 6]]) as u32;