ScanOptions: add disable_keydb to skip all keydb lookup

A caller that resolves Unit Keys out-of-band can now set disable_keydb
so the scan consults no keydb at all — neither an explicit keydb_path
nor the standard search locations. Without it, a keydb that merely sits
in a default location ('~/.config/...') silently shadows the out-of-band
path. unit_key still takes precedence over everything.

src/mux/resolve.rs

@@ -187,6 +187,7 @@ pub fn input(url: &str, opts: &InputOptions) -> io::Result<Box<dyn crate::pes::S
             let scan_opts = crate::disc::ScanOptions {
                 keydb_path: opts.keydb_path.as_ref().map(Into::into),
                 unit_key: opts.unit_key,
+                ..Default::default()
             };
             // FileSectorSource is the sole file-backed sector source.
             // It carries the platform-tuned SEQUENTIAL fadvise hint