Fix the hardlink count decrement threshold and unlink cleanup (#610)

- Fixes a hardlink count decrement threshold
- Fixes unlinking not freeing the first inode

Resolves the failing added tests:

```
✘ Test hardlinkLinksCount() recorded an issue at TestEXT4Format+Link.swift:43:9: Expectation failed: try EXT4.EXT4Reader(blockDevice: afterUnlink).stat("/original").inode.linksCount == 1
✘ Test hardlinkLinksCount() failed after 0.016 seconds with 1 issue.
```

```
✘ Test unlinkFirstInodeFreesInode() recorded an issue at TestEXT4Format+Link.swift:58:9: Expectation failed: try EXT4.EXT4Reader(blockDevice: path).superBlock.freeInodesCount == EXT4.EXT4Reader(blockDevice: emptyPath).superBlock.freeInodesCount
✘ Test unlinkFirstInodeFreesInode() failed after 0.014 seconds with 1 issue.
```

Author: dkovba

Sources/ContainerizationEXT4/EXT4+Formatter.swift

@@ -233,13 +233,13 @@ extension EXT4 {
                 // the file we are deleting is a hardlink, decrement the link count
                 let linkedInodePtr = self.inodes[Int(hardlink - 1)]
                 var linkedInode = linkedInodePtr.pointee
-                if linkedInode.linksCount > 2 {
+                if linkedInode.linksCount > 1 {
                     linkedInode.linksCount -= 1
                     linkedInodePtr.initialize(to: linkedInode)
                 }
             }
 
-            guard inodeNumber > FirstInode else {
+            guard inodeNumber >= FirstInode else {
                 // Free the inodes and the blocks related to the inode only if its valid
                 return
             }

Tests/ContainerizationEXT4Tests/TestEXT4Format+Create.swift

@@ -78,33 +78,3 @@ struct Ext4FormatCreateTests {
         }  // should create /parent automatically
     }
 }
-
-@Suite(.serialized)
-struct NegativeTimestampRoundtripTests {
-    private let fsPath = FilePath(
-        FileManager.default.temporaryDirectory
-            .appendingPathComponent("ext4-pre1970-roundtrip.img", isDirectory: false))
-    private let apollo11MoonLanding: Date = {
-        let f = ISO8601DateFormatter()
-        f.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
-        return f.date(from: "1969-07-20T20:17:39.9Z")!
-    }()
-
-    @Test func encodeNegativeTimestamp() throws {
-        let formatter = try EXT4.Formatter(fsPath, minDiskSize: 32.kib())
-        defer { try? formatter.close() }
-        let ts = FileTimestamps(access: apollo11MoonLanding, modification: apollo11MoonLanding, creation: apollo11MoonLanding)
-        try formatter.create(path: FilePath("/file"), mode: EXT4.Inode.Mode(.S_IFREG, 0o755), ts: ts, buf: nil)
-    }
-
-    @Test func decodeNegativeTimestamp() throws {
-        let reader = try EXT4.EXT4Reader(blockDevice: fsPath)
-        let (_, inode) = try reader.stat(FilePath("/file"))
-        let mtime = Date(fsTimestamp: UInt64(inode.mtime) | (UInt64(inode.mtimeExtra) << 32))
-        let atime = Date(fsTimestamp: UInt64(inode.atime) | (UInt64(inode.atimeExtra) << 32))
-        let crtime = Date(fsTimestamp: UInt64(inode.crtime) | (UInt64(inode.crtimeExtra) << 32))
-        #expect(mtime == apollo11MoonLanding)
-        #expect(atime == apollo11MoonLanding)
-        #expect(crtime == apollo11MoonLanding)
-    }
-}

Tests/ContainerizationEXT4Tests/TestEXT4Format+Link.swift

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the Containerization project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import SystemPackage
+import Testing
+
+@testable import ContainerizationEXT4
+
+struct Ext4FormatLinkTests {
+    @Test func hardlinkLinksCount() throws {
+        func makeFile(unlink: Bool) throws -> FilePath {
+            let path = FilePath(
+                FileManager.default.temporaryDirectory
+                    .appendingPathComponent(UUID().uuidString, isDirectory: false))
+            let fmt = try EXT4.Formatter(path, minDiskSize: 32.kib())
+            try fmt.create(path: "/original", mode: EXT4.Inode.Mode(.S_IFREG, 0o755), buf: nil)
+            try fmt.link(link: "/hardlink", target: "/original")
+            if unlink {
+                try fmt.unlink(path: "/hardlink")
+            }
+            try fmt.close()
+            return path
+        }
+
+        let afterLink = try makeFile(unlink: false)
+        #expect(try EXT4.EXT4Reader(blockDevice: afterLink).stat("/original").inode.linksCount == 2)
+
+        let afterUnlink = try makeFile(unlink: true)
+        #expect(try EXT4.EXT4Reader(blockDevice: afterUnlink).stat("/original").inode.linksCount == 1)
+    }
+
+    @Test func unlinkFirstInodeFreesInode() throws {
+        let emptyPath = FilePath(FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString, isDirectory: false))
+        defer { try? FileManager.default.removeItem(at: emptyPath.url) }
+        try EXT4.Formatter(emptyPath, minDiskSize: 32.kib()).close()
+
+        let path = FilePath(FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString, isDirectory: false))
+        defer { try? FileManager.default.removeItem(at: path.url) }
+        let fmt = try EXT4.Formatter(path, minDiskSize: 32.kib())
+        try fmt.create(path: FilePath("/file"), mode: EXT4.Inode.Mode(.S_IFREG, 0o755), buf: nil)
+        try fmt.unlink(path: FilePath("/file"))
+        try fmt.close()
+
+        #expect(try EXT4.EXT4Reader(blockDevice: path).superBlock.freeInodesCount == EXT4.EXT4Reader(blockDevice: emptyPath).superBlock.freeInodesCount)
+    }
+}

Tests/ContainerizationEXT4Tests/TestEXT4Format.swift

@@ -220,6 +220,36 @@ struct Ext4FormatTests: ~Copyable {
     }
 }
 
+@Suite(.serialized)
+struct NegativeTimestampRoundtripTests {
+    private let fsPath = FilePath(
+        FileManager.default.temporaryDirectory
+            .appendingPathComponent("ext4-pre1970-roundtrip.img", isDirectory: false))
+    private let apollo11MoonLanding: Date = {
+        let f = ISO8601DateFormatter()
+        f.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+        return f.date(from: "1969-07-20T20:17:39.9Z")!
+    }()
+
+    @Test func encodeNegativeTimestamp() throws {
+        let formatter = try EXT4.Formatter(fsPath, minDiskSize: 32.kib())
+        defer { try? formatter.close() }
+        let ts = FileTimestamps(access: apollo11MoonLanding, modification: apollo11MoonLanding, creation: apollo11MoonLanding)
+        try formatter.create(path: FilePath("/file"), mode: EXT4.Inode.Mode(.S_IFREG, 0o755), ts: ts, buf: nil)
+    }
+
+    @Test func decodeNegativeTimestamp() throws {
+        let reader = try EXT4.EXT4Reader(blockDevice: fsPath)
+        let (_, inode) = try reader.stat(FilePath("/file"))
+        let mtime = Date(fsTimestamp: UInt64(inode.mtime) | (UInt64(inode.mtimeExtra) << 32))
+        let atime = Date(fsTimestamp: UInt64(inode.atime) | (UInt64(inode.atimeExtra) << 32))
+        let crtime = Date(fsTimestamp: UInt64(inode.crtime) | (UInt64(inode.crtimeExtra) << 32))
+        #expect(mtime == apollo11MoonLanding)
+        #expect(atime == apollo11MoonLanding)
+        #expect(crtime == apollo11MoonLanding)
+    }
+}
+
 struct Ext4FormatEmptyRangeTests {
     @Test func closeEmptyFilesystem() throws {
         let fsPath = FilePath(