Initial Go SDK for Keysat licensing service

Pure-Go, stdlib-only implementation of the LIC1 wire format:
- ParseKey + Verify + ParseAndVerify for offline verification
- HashFingerprint helper (SHA-256, matching the daemon's contract)
- LoadPublicKeyPEM for the standard PKIX-encoded Ed25519 public keys
  the daemon emits
- Client.Validate / Client.PublicKey for online checks against a
  running Keysat daemon
- LicensePayload struct with idiomatic Go getters (IsTrial,
  IsFingerprintBound, IsExpiredAt, HasEntitlement)

Wire-format crosscheck against the shared tests/crosscheck/vector.json
(the same file the Rust, TypeScript, Python SDKs and the daemon itself
test against). All four fixtures pass — v1 legacy fingerprint-bound,
v2 trial with entitlements, v2 perpetual unbound, plus end-to-end
PEM-load → ParseAndVerify signature roundtrip. Confirms byte-for-byte
agreement across five independent implementations.

No third-party dependencies. Module path:
github.com/keysat-xyz/keysat-client-go
go 1.21

examples/offline_verify.go

@@ -0,0 +1,34 @@
+// Offline verification example — `go run examples/offline_verify.go`
+// from the package root. Replace the embedded pubkey + license key
+// with your own.
+package main
+
+import (
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/keysat-xyz/keysat-client-go"
+)
+
+const publicKeyPEM = `-----BEGIN PUBLIC KEY-----
+MCowBQYDK2VwAyEAA6EHv/POEL4dcN0Y50vAmWfk1jCbpQ1fHdyGZBJVMbg=
+-----END PUBLIC KEY-----`
+
+const licenseKey = `LIC1-AIBW6RVE6YGS6SRIW2VD5D57N4UPBKVKVKVLXO6MZTO533XO53XO53QAAAAAAZKT6EAAAAAAABYT7MYA2NCGD73DC4G6MM5VVISRFTROCWWBECY4GJNM3LNGPQBOLFF2HM6QEA3QOJXQY3LVNR2GSLLEMV3GSY3F-QPSJIDYL6Y5TFCKXQ2SN43EDJIZIRJZCEROM2I4MJHODT6KO4KDPW6AJ3HMYJERYPD34CF2Z46PXPYFKSRZS7BDZKVKWE57UBJSTEBI`
+
+func main() {
+	pub, err := keysat.LoadPublicKeyPEM(publicKeyPEM)
+	if err != nil {
+		log.Fatalf("loading public key: %v", err)
+	}
+	payload, err := keysat.ParseAndVerify(licenseKey, pub)
+	if err != nil {
+		log.Fatalf("license invalid: %v", err)
+	}
+	fmt.Printf("OK — version=%d trial=%v fingerprint_bound=%v entitlements=%v\n",
+		payload.Version, payload.IsTrial(), payload.IsFingerprintBound(), payload.Entitlements)
+	if payload.IsExpiredAt(time.Now().Unix()) {
+		fmt.Println("(expired)")
+	}
+}