Add lyng.legacy_digest module with LegacyDigest.sha1()

Provides a pure Kotlin/KMP SHA-1 implementation with no extra dependencies,
exposed as `LegacyDigest.sha1(data)` in the `lyng.legacy_digest` package.

The naming deliberately signals that SHA-1 is cryptographically broken:
the object name `LegacyDigest` and prominent doc-comment warnings steer
users away from security-sensitive use while still enabling interoperability
with legacy protocols and file formats that require SHA-1.

API:
  import lyng.legacy_digest
  val hex: String = LegacyDigest.sha1("some string")   // UTF-8 input
  val hex: String = LegacyDigest.sha1(someBuffer)       // raw-byte input

Tests cover FIPS 180-4 known-answer vectors (empty, "abc", long message,
quick-brown-fox) at both the Kotlin and Lyng integration levels.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lynglib/src/commonTest/kotlin/net/sergeych/lyng/LegacyDigestTest.kt

@@ -0,0 +1,113 @@
+/*
+ * Copyright 2026 Sergey S. Chernov real.sergeych@gmail.com
+ *
+ * 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
+ *
+ *     http://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.
+ *
+ */
+
+package net.sergeych.lyng
+
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class LegacyDigestTest {
+
+    // --- Kotlin-level unit tests for the SHA-1 implementation ---
+
+    @Test
+    fun sha1KotlinEmptyString() {
+        // SHA-1("") = da39a3ee5e6b4b0d3255bfef95601890afd80709
+        assertEquals(
+            "da39a3ee5e6b4b0d3255bfef95601890afd80709",
+            LegacyDigest.sha1Hex(ByteArray(0))
+        )
+    }
+
+    @Test
+    fun sha1KotlinAbc() {
+        // SHA-1("abc") = a9993e364706816aba3e25717850c26c9cd0d89d
+        assertEquals(
+            "a9993e364706816aba3e25717850c26c9cd0d89d",
+            LegacyDigest.sha1Hex("abc".encodeToByteArray())
+        )
+    }
+
+    @Test
+    fun sha1KotlinLongerMessage() {
+        // SHA-1("abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq")
+        // = 84983e441c3bd26ebaae4aa1f95129e5e54670f1
+        val msg = "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"
+        assertEquals(
+            "84983e441c3bd26ebaae4aa1f95129e5e54670f1",
+            LegacyDigest.sha1Hex(msg.encodeToByteArray())
+        )
+    }
+
+    @Test
+    fun sha1KotlinExactlyOneBlock() {
+        // "The quick brown fox jumps over the lazy dog"
+        // SHA-1 = 2fd4e1c67a2d28fced849ee1bb76e7391b93eb12
+        val msg = "The quick brown fox jumps over the lazy dog"
+        assertEquals(
+            "2fd4e1c67a2d28fced849ee1bb76e7391b93eb12",
+            LegacyDigest.sha1Hex(msg.encodeToByteArray())
+        )
+    }
+
+    // --- Lyng-level integration tests ---
+
+    @Test
+    fun sha1LyngStringInput() = runTest {
+        eval(
+            """
+            import lyng.legacy_digest
+            assertEquals(
+                "a9993e364706816aba3e25717850c26c9cd0d89d",
+                LegacyDigest.sha1("abc")
+            )
+            assertEquals(
+                "da39a3ee5e6b4b0d3255bfef95601890afd80709",
+                LegacyDigest.sha1("")
+            )
+            """.trimIndent()
+        )
+    }
+
+    @Test
+    fun sha1LyngBufferInput() = runTest {
+        eval(
+            """
+            import lyng.legacy_digest
+            import lyng.buffer
+            val buf = Buffer.decodeHex("616263")  // "abc" in hex
+            assertEquals(
+                "a9993e364706816aba3e25717850c26c9cd0d89d",
+                LegacyDigest.sha1(buf)
+            )
+            """.trimIndent()
+        )
+    }
+
+    @Test
+    fun sha1LyngReturnType() = runTest {
+        eval(
+            """
+            import lyng.legacy_digest
+            val h = LegacyDigest.sha1("hello")
+            assert(h is String)
+            assertEquals(40, h.length)
+            """.trimIndent()
+        )
+    }
+}