Make iso:// a destination for any image source, not just a drive

Writing an iso:// meant a raw sector copy off a drive, so an existing
image could only ever be a source. Decrypting one you already had meant
finding the disc again.

The engine's copy is the recovery path — mapfile, multipass, damage-jump,
auto-resume — and all of it exists because optical media returns read
errors. A file does not, so a non-drive source gets write_image instead:
sectors in, bytes out, once, no recovery machinery. Keeping them apart is
not just tidiness. The mapfile identity check compares AACS unit keys and
the VID, both empty for an already-decrypted source, so identity passes
for any such source — a second run with different input to the same
output path would resume over the previous image and report success.

A short read is an error rather than a zero-fill: padding a truncated
source yields an image that looks complete and is not, which is the worst
outcome for a copy someone means to keep.

src/error.rs

@@ -243,6 +243,8 @@ pub const E_SYNC_WORKER_LOST: u16 = 9057;
 /// READ CAPACITY returned a short or overflowing transfer.
 pub const E_DISC_CAPACITY_MALFORMED: u16 = 9047;
 pub const E_DRIVE_INQUIRY_SHORT: u16 = 9058;
+pub const E_SHORT_IMAGE_READ: u16 = 9059;
+pub const E_EMPTY_IMAGE: u16 = 9060;
 
 // ── Error enum ──────────────────────────────────────────────────────────────
 
@@ -371,6 +373,22 @@ pub enum Error {
         index: usize,
         count: usize,
     },
+    /// An image-level write read fewer bytes than the sector count it asked for.
+    ///
+    /// Distinct from [`Error::IoError`] on purpose: the read SUCCEEDED and simply
+    /// returned less than a whole sector run, which for a file-backed source means
+    /// the source is shorter than its declared capacity. Zero-filling the gap
+    /// would produce an image that looks complete and is not — the worst outcome
+    /// for a copy someone intends to keep — so it is an error instead.
+    ShortImageRead {
+        lba: u32,
+        expected: u32,
+        got: u32,
+    },
+    /// An image-level write was asked for zero sectors. A zero-byte image is
+    /// never the intent, and reporting it here names the problem at the point it
+    /// is knowable rather than leaving an empty file behind.
+    EmptyImage,
     IfoParse,
     /// A title produced NO muxable frames: the mux driver's pump ended without
     /// any video track's `codec_private` resolving, or the MKV muxer reached
@@ -744,6 +762,8 @@ impl Error {
             Error::UdfNotFilesystem => E_UDF_NOT_FILESYSTEM,
             Error::UdfBufferTooSmall => E_UDF_BUFFER_TOO_SMALL,
             Error::DiscTitleRange { .. } => E_DISC_TITLE_RANGE,
+            Error::ShortImageRead { .. } => E_SHORT_IMAGE_READ,
+            Error::EmptyImage => E_EMPTY_IMAGE,
             Error::IfoParse => E_IFO_PARSE,
             Error::MkvInvalid => E_MKV_INVALID,
             Error::MkvSourceInvalid => E_MKV_SOURCE_INVALID,