Split a JWT, decode header + payload (base64url), print 'alg=...' and 'sub=...'.
A JSON Web Token has three base64url segments separated by dots: `header.payload.signature`.
Read ONE JWT line from stdin. Decode the header and payload (remember base64url needs padding: `s + "=" * (-len(s) % 4)`). Parse them as JSON. Print exactly two lines:
```
alg=<header.alg>
sub=<payload.sub>
```
Do NOT verify the signature — this is a recon task, not authentication.
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