Fix GetRawUnit for v1 MPR files: convert UUID to GUID blob

GetRawUnit passed the UUID string directly to SQLite for v1 MPRs,
but UnitID is stored as a 16-byte GUID blob. The query never matched,
silently returning "no rows" for every unit lookup on v1 projects.

Use types.UUIDToBlob() before querying, consistent with
getUnitByIDV1() in reader_units.go and GetRawUnitBytes() in
writer_security.go which already have the correct implementation.

Fixes #705

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: densmoe

sdk/mpr/reader_documents.go

@@ -462,8 +462,9 @@ func (r *Reader) GetRawUnit(id model.ID) (map[string]any, error) {
 			return nil, fmt.Errorf("failed to read unit contents: %w", err)
 		}
 	} else {
-		// V1: Read from database
-		row := r.db.QueryRow("SELECT Contents FROM Unit WHERE UnitID = ?", string(id))
+		// V1: Read from database — convert UUID to GUID blob for the query
+		unitIDBlob := types.UUIDToBlob(string(id))
+		row := r.db.QueryRow("SELECT Contents FROM Unit WHERE UnitID = ?", unitIDBlob)
 		err = row.Scan(&contents)
 		if err != nil {
 			return nil, fmt.Errorf("failed to read unit from database: %w", err)