feat(mobile): reconstruir+ampliar binding gomobile para paridad con la web

El wrapper mobile/unibus.go se habia perdido del repo (solo quedaba compilado
en el .aar del 5 jun). Se reconstruye y amplia con:

- Wallet BIP39 determinista: NewMnemonic, ValidateMnemonic, DeriveAndSaveIdentity.
  Deriva la MISMA identidad que uniweb (web/src/wallet/derive.ts): PBKDF2-BIP39 ->
  HKDF-SHA256(info unibus-sign-v1 / unibus-kex-v1) -> Ed25519 + X25519. Test de
  paridad contra el vector de oro (mnemonica abandon...about -> sign_pub
  34302746...b3c8) garantiza misma cuenta web<->movil byte a byte.
- Selector de salas: Session.ListMyRooms() -> JSON [{id,subject,mode,role}].
- Nombres legibles: Session.Directory() + Client.Directory()/EndpointID() nuevos
  en pkg/client (GET /directory firmado).
- HasIdentity/SignPubAt para el onboarding.

Aditivo; build/vet/test del modulo verdes (incluido TestDeriveParityWithWeb).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mobile/derive_test.go

@@ -0,0 +1,49 @@
+package mobile
+
+import (
+	"encoding/hex"
+	"testing"
+)
+
+// TestDeriveParityWithWeb pins the Go wallet derivation to the TypeScript one
+// (web/src/wallet/derive.ts). The canonical BIP39 test mnemonic must derive to this
+// exact Ed25519 sign_pub — the same value the uniweb client showed for this phrase.
+// If this fails, web and mobile would derive different identities from the same seed
+// and the "same account on both devices" guarantee breaks.
+func TestDeriveParityWithWeb(t *testing.T) {
+	const mnemonic = "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
+	const wantSignPub = "34302746268e7370d35940e1bcef8c0b1c13a857ea6209e6ecc6e9b3af06b3c8"
+
+	id, err := deriveIdentity(mnemonic)
+	if err != nil {
+		t.Fatalf("deriveIdentity: %v", err)
+	}
+	if got := hex.EncodeToString(id.SignPub); got != wantSignPub {
+		t.Fatalf("sign_pub mismatch:\n got  %s\n want %s", got, wantSignPub)
+	}
+	if len(id.SignPriv) != 64 || len(id.KexPub) != 32 || len(id.KexPriv) != 32 {
+		t.Fatalf("bad key lengths: signPriv=%d kexPub=%d kexPriv=%d",
+			len(id.SignPriv), len(id.KexPub), len(id.KexPriv))
+	}
+	// sign_priv is Go's ed25519 layout: seed || pub, so its tail must equal sign_pub.
+	if got := hex.EncodeToString(id.SignPriv[32:]); got != wantSignPub {
+		t.Fatalf("sign_priv tail != sign_pub:\n got  %s\n want %s", got, wantSignPub)
+	}
+}
+
+// TestMnemonicRoundTrip checks a freshly generated phrase validates and derives.
+func TestMnemonicRoundTrip(t *testing.T) {
+	m, err := NewMnemonic()
+	if err != nil {
+		t.Fatalf("NewMnemonic: %v", err)
+	}
+	if !ValidateMnemonic(m) {
+		t.Fatalf("generated mnemonic failed validation: %q", m)
+	}
+	if _, err := deriveIdentity(m); err != nil {
+		t.Fatalf("deriveIdentity(fresh): %v", err)
+	}
+	if ValidateMnemonic("not a real mnemonic at all please") {
+		t.Fatal("garbage phrase validated as a mnemonic")
+	}
+}