feat(fls): Java's view of the e-mail programs

The Java view now reads what FILIUS 2.14 keeps for its e-mail programs:
the server's domain, switch and bean accounts with their mailboxes, and the
accounts of /mailserver/konten.txt, which Java reads at every simulation
start and which so win over the bean list; the client's three folders and
address book, and its account from /konten.txt (no bean holds it).

javaEmail.ts ports the bean setters and both file readers
(stringZuListeBenutzerkonten, EmailKonto(String)); javaString.ts Java's
trim, split and StringTokenizer; javaFileSystem.ts the file lookup, now
shared with the DNS server, with the four non-ASCII units Java's
equalsIgnoreCase also takes for an ASCII letter (measured over the BMP).

Checked against Java on the corpus (the oracle dumps the e-mail state) and
on 43 account files read by EmailServer.kontenLaden and
EmailAnwendung.laden (EmailFileCases). Found: older files keep recipients
as strings in the typed lists; a getter copy of the sender is filled and
set back; one short server line makes Java throw for the whole file; a
message with an empty text is lost on reading; a broken last client line
empties the account.

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

packages/fls/src/javaFileSystem.ts

@@ -0,0 +1,51 @@
+// How Java FILIUS finds a file by path (Dateisystem.holeDatei): the first
+// node in preorder whose path equals the path ignoring case, and only if that
+// node is a file.
+
+import type { FlsFileTree } from './systemSoftware.js';
+
+/**
+ * The content of the file at an ASCII path as `Dateisystem.holeDatei` finds
+ * it. Undefined when there is none; a file without content reads as empty
+ * (Java would fail on it).
+ */
+export function javaFileContent(root: FlsFileTree | undefined, asciiPath: string): string | undefined {
+  if (root === undefined) return undefined;
+  const found = firstInPreorder(root, '', (path) => javaEqualsIgnoreCase(path, asciiPath));
+  return found?.kind === 'file' ? (found.content ?? '') : undefined;
+}
+
+/** Preorder below the root; a node's path is "/" plus each name after the root's. */
+function firstInPreorder(node: FlsFileTree, path: string, matches: (path: string) => boolean): FlsFileTree | undefined {
+  for (const child of node.children) {
+    const childPath = `${path}/${child.kind === 'other' ? child.className : String(child.name)}`;
+    if (matches(childPath)) return child;
+    const found = firstInPreorder(child, childPath, matches);
+    if (found !== undefined) return found;
+  }
+  return undefined;
+}
+
+// Besides ASCII case, String.equalsIgnoreCase takes these BMP units for an
+// ASCII letter (measured on the JDK over every unit): dotted and dotless I,
+// the Kelvin sign and the long s.
+const JAVA_ASCII_CASE_FOLDS = new Map([
+  ['\u0130', 'i'],
+  ['\u0131', 'i'],
+  ['\u212A', 'k'],
+  ['\u017F', 's'],
+]);
+
+/** `String.equalsIgnoreCase` against an ASCII path, unit by unit. */
+function javaEqualsIgnoreCase(text: string, asciiPath: string): boolean {
+  if (text.length !== asciiPath.length) return false;
+  for (let index = 0; index < text.length; index++) {
+    if (asciiFold(text[index]!) !== asciiFold(asciiPath[index]!)) return false;
+  }
+  return true;
+}
+
+function asciiFold(unit: string): string {
+  if (unit >= 'A' && unit <= 'Z') return unit.toLowerCase();
+  return JAVA_ASCII_CASE_FOLDS.get(unit) ?? unit;
+}