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>

runtime/internal/lib/hash/maphash/maphash.go

@@ -18,6 +18,11 @@ package maphash
 
 import "unsafe"
 
+// NOTE: The following functions are not yet implemented and will panic with "intrinsic":
+// - Comparable(seed Seed, v any) uint64
+// - (*Hash).WriteComparable(v any) (int, error)
+// These functions require runtime intrinsic support that is not currently available.
+
 //go:linkname runtime_rand github.com/goplus/llgo/runtime/internal/runtime.fastrand64
 func runtime_rand() uint64