fix: ECDSA JWK thumbprint uses generator point instead of public key coordinates

[mesaglio]

getRFC7638Thumbprint was using params.Gx/Gy (the curve's generator point, a constant shared by all keys on the same curve) instead of pubkey.X/Y (the actual public key coordinates).
This caused every ECDSA key on P-256 to produce the identical kid, breaking the disable_legacy_key_id feature and making key rotation undetectable for affected deployments.

To reproduce

1. Generate two independent ECDSA P-256 key pairs

# First key pair
openssl ecparam -name prime256v1 -genkey -noout -out ecdsa1.key
openssl req -new -x509 -key ecdsa1.key -out ecdsa1.crt -days 365 -subj "/CN=docker-auth"

# Second key pair
openssl ecparam -name prime256v1 -genkey -noout -out ecdsa2.key
openssl req -new -x509 -key ecdsa2.key -out ecdsa2.crt -days 365 -subj "/CN=docker-auth-2"

2. Start the auth server using the first key pair (config.yml)

server:
  addr: ":5001"
  certificate: "ecdsa1.crt"
  key: "ecdsa1.key"

token:
  issuer: "Acme auth server"
  expiration: 900
  disable_legacy_key_id: true

users:
  "admin":
    password: "$2y$05$LO.vzwpWC5LZGqThvEfznu8qhb5SGqvBSWY1J3yZ4AxtMRZ3kN5jC"  # badmin

acl:
  - match: {account: "admin"}
    actions: ["*"]

3. Request a token and inspect the kid in the JWT header

TOKEN=$(curl -sk -u admin:badmin \
  "https://localhost:5001/auth?service=registry&scope=repository:test:pull" \
  | jq -r '.token')

echo $TOKEN | cut -d. -f1 \
  | awk '{ n=length($0)%4; if(n==2) print $0"=="; else if(n==3) print $0"="; else print $0 }' \
  | base64 -d | jq .

4. Repeat with the second key pair

Stop the server, update config.yml to use ecdsa2.crt/ecdsa2.key, restart, and request a new token.

---

Expected result (correct behavior, RFC 7638 compliant):

Each key produces a unique kid derived from its own public coordinates:
key 1 → kid: "GUebkAAx1QFg2mhM0v9rcdvJQPUacaez5c5bzQuf_eM"
key 2 → kid: "ho2fTx0KnTD6wdA8qyAfIxvL27hxCeubWu9suUh6XcI"

Actual result (buggy behavior, before this fix):

Both keys produce the identical kid because params.Gx/params.Gy (the P-256 generator point, a constant shared by all keys on the curve) are used instead of pubkey.X/pubkey.Y:
key 1 → kid: "xx0BcA-wMohw8atYDJOe6peGModklG2wRHBlXHMvl0M"
key 2 → kid: "xx0BcA-wMohw8atYDJOe6peGModklG2wRHBlXHMvl0M" ← identical