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

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Keysat
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.