fix(fls): archive path folding measured against Swift

41 path pairs replayed in the Swift core decide which spellings name
the same file. Two corrections: full Unicode case folding (ß, ligatures,
final sigma, micro, long s) instead of toLowerCase, and width folding of
the whole Halfwidth and Fullwidth Forms block. Swift treats composed and
decomposed spellings as one dictionary key; here the writer refuses both.

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

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

@@ -291,8 +291,47 @@ func readCorpus(root: URL) throws -> JSON {
     return result
 }
 
+// MARK: - Archive path folding
+
+struct PathCase: Decodable { let name: String; let first: String; let second: String }
+
+/// Whether Swift's archive writer treats two entry paths as the same file.
+/// The folding itself is file-private in Swift; the export reports it as a
+/// duplicate entry when two carried entries collide.
+func foldPaths(_ cases: [PathCase]) -> [JSON] {
+    cases.map { item in
+        var result: JSON = ["name": item.name]
+        // Swift compares strings by canonical equivalence: "é" and "e\u{301}"
+        // are one dictionary key, so they never reach the archive writer.
+        if item.first == item.second {
+            result["outcome"] = "sameKey"
+            return result
+        }
+        do {
+            _ = try TopologyProjectStore.exportFiliusArchiveWithReport(
+                from: TopologyEditorState(),
+                supplementalEntries: [item.first: Data("1".utf8), item.second: Data("2".utf8)]
+            )
+            result["outcome"] = "distinct"
+        } catch let error as TopologyFLSArchiveError {
+            result["outcome"] = error.code == .duplicateEntry ? "collide" : error.code.rawValue
+        } catch {
+            result["outcome"] = "error: \(error)"
+        }
+        return result
+    }
+}
+
 // MARK: - Main
 
+if CommandLine.arguments.count == 4, CommandLine.arguments[1] == "--path-folding" {
+    let cases = try JSONDecoder().decode([PathCase].self, from: Data(contentsOf: URL(fileURLWithPath: CommandLine.arguments[2])))
+    let data = try JSONSerialization.data(withJSONObject: foldPaths(cases), options: [.prettyPrinted, .sortedKeys])
+    try (data + Data("\n".utf8)).write(to: URL(fileURLWithPath: CommandLine.arguments[3]))
+    print("path folding: \(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])