Use SDK ParadexEvm for EVM auth; require paradex_py >= 0.6.3

[amayvs]

Summary

• _evm_authed_paradex now returns the SDK's ParadexEvm on standard environments (paradex_py 0.6.3 ships the full v2 SIWE flow incl. vault-operator targeting) — gaining onboarding-if-needed, JWT refresh on expiry, and the SDK's operator guards (e.g. refusing a pre-feature server that answers with the main account).
• The CLI's own v2 SIWE flow remains only as the --siwe-domain/--chain-id override fallback (e.g. --env local), which ParadexEvm cannot serve (per-env SIWE domains are hardcoded in the SDK). Unknown envs still get the helpful "pass --siwe-domain and --chain-id" error.
• paradex_py>=0.6.3 in pyproject + uv.lock.
• 3 new tests cover the routing (SDK path with operator forwarding, override fallback, unknown-env error); 94 pass locally.

Notes for reviewers

• The uv.lock entry for paradex-py 0.6.3 was written from PyPI's JSON API metadata (canonical URLs + sha256) because a local network security proxy blocks not-yet-vetted new packages from resolving; CI resolves normally.
• The guardian-command tests hit the live testnet API, which is currently scaled down (503) — if CI shows those 9 failing, re-run once testnet is back up.
• Follow-up candidate: upstream the SIWE subkey-registration payload builder into paradex-py, after which the CLI's remaining SIWE code can be deleted entirely.