feat(routing): canonical request hash

SHA-256 of (system, user) joined with 0x00 separator, truncated to
uint64. Drives deterministic sample-band routing: identical prompt pair
→ same hash → same local-vs-Claude decision on every call.

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

internal/routing/hash.go

@@ -0,0 +1,21 @@
+package routing
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+)
+
+// CanonicalHash returns a deterministic 64-bit hash of (system, user).
+// Used to make sample-band routing decisions reproducible: identical input
+// strings produce the same hash on every call, independent of process state.
+//
+// Inputs are joined with a 0x00 byte separator before hashing — distinguishes
+// (system="ab", user="cd") from (system="abcd", user="").
+func CanonicalHash(system, user string) uint64 {
+	h := sha256.New()
+	h.Write([]byte(system))
+	h.Write([]byte{0})
+	h.Write([]byte(user))
+	sum := h.Sum(nil)
+	return binary.BigEndian.Uint64(sum[:8])
+}