refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/cap/pow/prng.go

@@ -0,0 +1,56 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package pow
+
+import (
+	"fmt"
+	"strings"
+)
+
+// fnv1a returns the 32-bit FNV-1a hash of a string
+//
+//nolint:mnd // FNV-1a 算法位移常量
+func fnv1a(str string) uint32 {
+	var hash uint32 = 2166136261
+	for i := 0; i < len(str); i++ {
+		hash ^= uint32(str[i])
+		hash += (hash << 1) + (hash << 4) + (hash << 7) + (hash << 8) + (hash << 24)
+	}
+	return hash
+}
+
+// fnv1aResume resumes FNV-1a hashing from a given state
+//
+//nolint:mnd // FNV-1a 算法位移常量
+func fnv1aResume(state uint32, str string) uint32 {
+	h := state
+	for i := 0; i < len(str); i++ {
+		h ^= uint32(str[i])
+		h += (hashShift(h))
+	}
+	return h
+}
+
+// hashShift computes FNV-1a mix additions
+//
+//nolint:mnd
+func hashShift(h uint32) uint32 {
+	return (h << 1) + (h << 4) + (h << 7) + (h << 8) + (h << 24)
+}
+
+// prngFromHash generates a hex string of specified length using an initial hash state
+//
+//nolint:mnd // xorshift 算法位移常量
+func prngFromHash(initialHash uint32, length int) string {
+	state := initialHash
+	var result strings.Builder
+	for result.Len() < length {
+		state ^= state << 13
+		state ^= state >> 17
+		state ^= state << 5
+		hexStr := fmt.Sprintf("%08x", state)
+		result.WriteString(hexStr)
+	}
+	return result.String()[:length]
+}