feat(fls): DNS records from /dns/hosts, as Java's server answers them

Java's DNS server reads its records from the text file /dns/hosts on every
query. dnsHosts.ts ports that reading (DNSServer.leseRecordListe,
ResourceRecord, Dateisystem.holeDatei) and keeps in the model the records
Java answers: type exactly A, MX or NS, and a name Java accepted. The file
stays in the file system as it is.

Both oracles read 76 hand-picked hosts files (dns-hosts.cases.txt): the
Java oracle through DNSServer.holeResourceRecords (DnsHostsCases), the
Swift oracle through TopologyDNSHostsFileCodec (--dns-hosts). The Java view
matches Java on every case; the model matches Swift except for 33 named
cases in twelve reasons: Java splits at single spaces and LF only, has no
comments or "IN", turns a TTL that is no int into 3600, keeps duplicates,
and empties names with a leading digit. The Swift dump now carries each
server's records; the corpus comparison covers them on 16 files.

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

tools/swift-oracle/Sources/Oracle/main.swift

@@ -401,6 +401,7 @@ func describeSystem(_ state: TopologyEditorState, _ node: TopologyNode) -> JSON
         json["dnsServer"] = [
             "recursiveResolutionEnabled": dns.recursiveResolutionEnabled,
             "forwardingServerIPAddress": dns.forwardingServerIPAddress ?? NSNull(),
+            "records": dns.typedRecords.map { dnsRecordJSON($0) },
         ] as JSON
     }
     if let link = state.remoteLinkConfigurationsByNodeID[node.id] {
@@ -614,6 +615,32 @@ func foldPaths(_ cases: [PathCase]) -> [JSON] {
     }
 }
 
+// MARK: - /dns/hosts cases (packages/fls/test/dns-hosts.cases.txt)
+
+func dnsRecordJSON(_ record: TopologyDNSResourceRecord) -> [Any] {
+    [record.name.rawValue, record.type.rawValue, Int(record.ttlSeconds), record.target]
+}
+
+/// One case per line: a whole hosts file, ASCII with Java-style unicode
+/// escapes (backslash, u, four hex digits) — the file the Java oracle reads.
+func readDNSHostsCases(_ url: URL) throws -> [(line: String, text: String)] {
+    let lines = try String(contentsOf: url, encoding: .ascii).components(separatedBy: "\n")
+    return lines.dropLast().map { line in
+        var units: [UInt16] = []
+        var rest = Substring(line)
+        while let first = rest.first {
+            if rest.hasPrefix("\\u"), rest.count >= 6, let unit = UInt16(rest.dropFirst(2).prefix(4), radix: 16) {
+                units.append(unit)
+                rest = rest.dropFirst(6)
+            } else {
+                units.append(contentsOf: String(first).utf16)
+                rest = rest.dropFirst()
+            }
+        }
+        return (line, String(decoding: units, as: UTF16.self))
+    }
+}
+
 // MARK: - Main
 
 if CommandLine.arguments.count == 4, CommandLine.arguments[1] == "--path-folding" {
@@ -641,6 +668,15 @@ if CommandLine.arguments.count == 4, CommandLine.arguments[1] == "--strings" {
     exit(0)
 }
 
+if CommandLine.arguments.count == 4, CommandLine.arguments[1] == "--dns-hosts" {
+    let cases = try readDNSHostsCases(URL(fileURLWithPath: CommandLine.arguments[2]))
+    let output: [[Any]] = cases.map { [$0.line, TopologyDNSHostsFileCodec.records(from: $0.text).map { dnsRecordJSON($0) }] }
+    let data = try JSONSerialization.data(withJSONObject: output, options: [.prettyPrinted, .sortedKeys])
+    try (data + Data("\n".utf8)).write(to: URL(fileURLWithPath: CommandLine.arguments[3]))
+    print("dns hosts: \(cases.count) cases")
+    exit(0)
+}
+
 if CommandLine.arguments.count == 4, CommandLine.arguments[1] == "--corpus" {
     let summary = try readCorpus(root: URL(fileURLWithPath: CommandLine.arguments[2]))
     let data = try JSONSerialization.data(withJSONObject: summary, options: [.prettyPrinted, .sortedKeys])