Gradle: Replace legacy buildSrc mechanism with included build (#1524)

Motivation:
buildSrc is a special-case legacy mechanism.
Gradle recommends using an included build named build-logic instead:

https://docs.gradle.org/current/userguide/best_practices_structuring_builds.html#favor_composite_builds

Changes:
- Rename buildSrc/ to build-logic/
  - triggers reformatting
- Replace occurrences of "buildSrc" with "build-logic"
- Include the build-logic build in the main build (via
settings.gradle.kts)
- Apply convention plugins via plugin IDs instead of type-safe accessors
  - small tradeoff compared to buildSrc

Result:
- Faster and more isolated builds
- Build logic behaves like a normal build, making it easier to evolve
and reason about

---------

Co-authored-by: Daniel Chao <dan.chao@apple.com>

build-logic/src/main/kotlin/RevertYearOnlyChanges.kt

@@ -0,0 +1,77 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * 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 com.diffplug.spotless.FormatterFunc
+import com.diffplug.spotless.FormatterStep
+import java.io.File
+import java.io.Serial
+import java.io.Serializable
+
+/**
+ * A Spotless [FormatterStep] that suppresses formatting changes where the only difference between
+ * the formatted output and the file's content in the upstream base ref is the license header year.
+ *
+ * Avoids an issue where, in the process of working on the codebase:
+ * 1. A file is modified.
+ * 2. Spotless formats the file, and also updates the copyright year.
+ * 3. The original modification is reverted.
+ * 4. Spotless formats the file again, but now the copyright year is the updated year.
+ */
+class RevertYearOnlyChangesStep(private val repoRoot: File, private val ratchetFrom: String) :
+  Serializable {
+  companion object {
+    @Serial private const val serialVersionUID: Long = 1L
+  }
+
+  fun create(): FormatterStep =
+    FormatterStep.createLazy(
+      "revertYearOnlyChanges",
+      { this },
+      { RevertYearOnlyChangesFunc(repoRoot, ratchetFrom) },
+    )
+}
+
+class RevertYearOnlyChangesFunc(private val repoRoot: File, private val ratchetFrom: String) :
+  FormatterFunc.NeedsFile, Serializable {
+  companion object {
+    @Serial private const val serialVersionUID: Long = 1L
+
+    // Matches "Copyright © 2024" or "Copyright © 2024-2025"
+    private val YEAR_REGEX = Regex("""(Copyright © )\d{4}(-\d{4})?""")
+  }
+
+  override fun applyWithFile(unix: String, file: File): String {
+    val relativePath = repoRoot.toPath().relativize(file.toPath()).toString()
+    val upstreamContent = gitShow(ratchetFrom, relativePath) ?: return unix
+    val normalizedRaw = YEAR_REGEX.replace(unix, "\$1YEAR")
+    val normalizedUpstream = YEAR_REGEX.replace(upstreamContent, "\$1YEAR")
+    return if (normalizedRaw == normalizedUpstream) {
+      // Only the year changed — return the upstream content
+      upstreamContent
+    } else {
+      unix
+    }
+  }
+
+  private fun gitShow(ref: String, path: String): String? {
+    val process =
+      ProcessBuilder("git", "show", "$ref:$path")
+        .directory(repoRoot)
+        .redirectErrorStream(true)
+        .start()
+    val output = process.inputStream.readBytes().toString(Charsets.UTF_8)
+    return if (process.waitFor() == 0) output.replace("\r\n", "\n") else null
+  }
+}