chore(shared): add hyperboloide/lk for licence signing

Probed the library before building against it. Two corrections to plan 1:

- PublicKey.ToB32String() returns one value, not (string, error)
- The scheme is ECDSA P-384 with SHA-256, not ed25519 as the spec and plan
  claim. Design is unaffected; the prose needs fixing.

shared/go.sum

@@ -4,6 +4,8 @@ github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
 github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/hyperboloide/lk v0.0.0-20251220053519-b291812e3216 h1:Luh+sE/W2M+V0Y+jlZN7nJefLNHc4/y93xxl+rFD7k0=
+github.com/hyperboloide/lk v0.0.0-20251220053519-b291812e3216/go.mod h1:/OLW9HZj6qtQ7gWTGwuO3JrUZ+MC7I7TLRuNl14TYuo=
 github.com/klauspost/compress v1.17.6 h1:60eq2E/jlfwQXtvZEeBUYADs+BwKBWURIY+Gj2eRGjI=
 github.com/klauspost/compress v1.17.6/go.mod h1:/dCuZOvVtNoHsyb+cuJD3itjs3NbnF6KH9zAO4BDxPM=
 github.com/xdg-go/pbkdf2 v1.0.0 h1:Su7DPu48wXMwC3bs7MCNG+z4FhcyEuz5dlvchbq0B0c=