test: expand maphash demo with comprehensive API coverage

- Add tests for all major hash/maphash public APIs:
  * Hash basics (WriteString, Sum64, Reset)
  * MakeSeed and SetSeed functionality
  * Write methods (Write, WriteByte, WriteString)
  * Bytes and String convenience functions
- Use panic() for unexpected errors instead of silent failures
- Add proper error checking and validation
- Document Comparable/WriteComparable limitations in overlay

Note: Comparable() and WriteComparable() are not yet supported and will
panic with 'intrinsic' error as they require runtime intrinsic support.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

_demo/go/maphash/maphash.go

@@ -6,18 +6,119 @@ import (
 )
 
 func main() {
-	var h maphash.Hash
-	h.WriteString("hello")
-	hash1 := h.Sum64()
-	fmt.Printf("0x%x\n", hash1)
-
-	h.Reset()
-	h.WriteString("world")
-	hash2 := h.Sum64()
-	fmt.Printf("0x%x\n", hash2)
-
-	h.Reset()
-	h.WriteString("hello")
-	hash3 := h.Sum64()
-	fmt.Printf("0x%x == 0x%x\n", hash1, hash3)
+	testHashBasics()
+	testMakeSeed()
+	testSetSeed()
+	testWriteMethods()
+	testBytes()
+	testString()
+}
+
+func testHashBasics() {
+	fmt.Println("=== Test Hash Basics ===")
+	var h maphash.Hash
+	n, err := h.WriteString("hello")
+	if err != nil {
+		panic(fmt.Sprintf("WriteString failed: %v", err))
+	}
+	if n != 5 {
+		panic(fmt.Sprintf("WriteString returned %d, expected 5", n))
+	}
+	hash1 := h.Sum64()
+	fmt.Printf("Hash of 'hello': 0x%x\n", hash1)
+
+	h.Reset()
+	n, err = h.WriteString("world")
+	if err != nil {
+		panic(fmt.Sprintf("WriteString failed: %v", err))
+	}
+	hash2 := h.Sum64()
+	fmt.Printf("Hash of 'world': 0x%x\n", hash2)
+
+	h.Reset()
+	n, err = h.WriteString("hello")
+	if err != nil {
+		panic(fmt.Sprintf("WriteString failed: %v", err))
+	}
+	hash3 := h.Sum64()
+	if hash1 != hash3 {
+		panic(fmt.Sprintf("Hash mismatch: 0x%x != 0x%x", hash1, hash3))
+	}
+	fmt.Printf("Hash consistency verified: 0x%x == 0x%x\n", hash1, hash3)
+}
+
+func testMakeSeed() {
+	fmt.Println("\n=== Test MakeSeed ===")
+	seed1 := maphash.MakeSeed()
+	seed2 := maphash.MakeSeed()
+	fmt.Printf("Seed 1: %v\n", seed1)
+	fmt.Printf("Seed 2: %v\n", seed2)
+	if seed1 == seed2 {
+		fmt.Println("Warning: Seeds are identical (rare but possible)")
+	}
+}
+
+func testSetSeed() {
+	fmt.Println("\n=== Test SetSeed ===")
+	var h1, h2 maphash.Hash
+	seed := maphash.MakeSeed()
+	
+	h1.SetSeed(seed)
+	n, err := h1.WriteString("test")
+	if err != nil {
+		panic(fmt.Sprintf("WriteString failed: %v", err))
+	}
+	hash1 := h1.Sum64()
+	
+	h2.SetSeed(seed)
+	n, err = h2.WriteString("test")
+	if err != nil {
+		panic(fmt.Sprintf("WriteString failed: %v", err))
+	}
+	hash2 := h2.Sum64()
+	
+	if hash1 != hash2 {
+		panic(fmt.Sprintf("Hashes with same seed should match: 0x%x != 0x%x", hash1, hash2))
+	}
+	fmt.Printf("Same seed produces same hash: 0x%x == 0x%x\n", hash1, hash2)
+}
+
+func testWriteMethods() {
+	fmt.Println("\n=== Test Write Methods ===")
+	var h maphash.Hash
+	
+	data := []byte("hello")
+	n, err := h.Write(data)
+	if err != nil {
+		panic(fmt.Sprintf("Write failed: %v", err))
+	}
+	if n != len(data) {
+		panic(fmt.Sprintf("Write returned %d, expected %d", n, len(data)))
+	}
+	hash1 := h.Sum64()
+	fmt.Printf("Hash after Write: 0x%x\n", hash1)
+	
+	h.Reset()
+	n, err = h.WriteByte('A')
+	if err != nil {
+		panic(fmt.Sprintf("WriteByte failed: %v", err))
+	}
+	hash2 := h.Sum64()
+	fmt.Printf("Hash after WriteByte('A'): 0x%x\n", hash2)
+}
+
+func testBytes() {
+	fmt.Println("\n=== Test Bytes Function ===")
+	seed := maphash.MakeSeed()
+	data := []byte("test data")
+	hash := maphash.Bytes(seed, data)
+	fmt.Printf("Bytes hash: 0x%x\n", hash)
+}
+
+func testString() {
+	fmt.Println("\n=== Test String Function ===")
+	seed := maphash.MakeSeed()
+	str := "test string"
+	hash := maphash.String(seed, str)
+	fmt.Printf("String hash: 0x%x\n", hash)
 }