feat(library): adopt SACD images found by the folder scan

The scan never treated an .iso as audio; images only entered the library
through Open > disc image. Each root's scan now walks its .iso files,
asks qbz_disc::sacd::is_sacd_image for the SACDMTOC signature (PVD plus
Master TOC sector, four 2 KB reads at most, no directory parsing) so a DVD
or data ISO leaves silently, skips images already imported with the same
size and mtime without any I/O, and adopts the rest through the
generation-based import. The row builder moved from sacd_qt into
qbz_library::sacd_scan and the manual open is now its client, with the
translated fallback labels. Parser and import rejections are scan errors.

cc

crates/qbz-library/Cargo.toml

@@ -27,6 +27,9 @@ dirs = "6"
 # Audio metadata
 lofty.workspace = true
 qbz-dsd = { path = "../qbz-dsd" }
+# SACD images found by the folder scan: signature sniff + Scarlet Book TOC
+# parse + the disc memory (sacd_scan.rs). qbz-disc depends on nothing here.
+qbz-disc = { path = "../qbz-disc" }
 
 # Image processing (thumbnails)
 image = { version = "0.25", default-features = false, features = ["png", "jpeg", "webp"] }