feat(doc): the field rule for a modem's code, which the sheet's commit left behind

87c09c6 uses rule "modemCode" and setModemCode through it; both lived in
these two files, which that commit did not take.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/doc/src/fields.ts

@@ -18,7 +18,7 @@
 // " 10.0.0.1"; a phone's keyboard puts a space after a word). A name and an
 // SSID are taken as typed, spaces included, as Java takes them.
 
-import { DIRECTLY_CONNECTED_SOURCE_MARKER, IP_ADDRESS_PATTERN, javaIpCheck, parseIPv4 } from '@mdg-filius/core';
+import { DIRECTLY_CONNECTED_SOURCE_MARKER, IP_ADDRESS_PATTERN, javaIpCheck, modemCodeOf, parseIPv4 } from '@mdg-filius/core';
 
 export type FieldRule =
   | 'name'
@@ -34,7 +34,9 @@ export type FieldRule =
   /** A firewall rule's mask: empty, or ones then zeros (EingabenUeberpruefung.isValidSubnetmask). */
   | 'firewallMask'
   /** A firewall rule's port: empty for all ports, or musterPort. */
-  | 'portOrEmpty';
+  | 'portOrEmpty'
+  /** The code of another project's modem (modemCodeOf: six characters, spaces, dashes and small letters allowed). */
+  | 'modemCode';
 
 /** What a field's text comes to: the value the device takes, or that it takes none. */
 export type CheckedField = { readonly value: string } | { readonly invalid: true };
@@ -97,6 +99,10 @@ export function checkField(rule: FieldRule, text: string): CheckedField {
     }
     case 'portOrEmpty':
       return text.trim() === '' ? { value: '' } : checkField('port', text);
+    case 'modemCode': {
+      const code = modemCodeOf(text);
+      return code === undefined ? INVALID : { value: code };
+    }
     case 'macAddress':
       // Upper case, as the model keeps MAC addresses (Java keeps them as typed and compares ignoring case).
       return MAC_ADDRESS_PATTERN.test(text.trim()) ? { value: text.trim().toUpperCase() } : INVALID;