fix(tls): derive OMS exporter secret from TLS key log (stdlib only)

[ale-rinaldi]

Summary

CPython's ssl module doesn't expose export_keying_material (RFC 5705), so _activate_tls always fell into Could not extract OMS exporter secret and set oms_secret = None — leaving password legitimation / HMAC key derivation as dead code against real PLCs (#718).

This derives the secret using only the standard library — no new dependency:

• Enable the TLS key log (SSLContext.keylog_filename) on a private 0600 temp file, removed immediately after the handshake.
• Read the TLS 1.3 exporter_master_secret from it and run the RFC 8446 exporter derivation with stdlib hmac/hashlib (_tls13_exporter).

Relationship to #772

This is an alternative to #772, which solves the same problem by adding a pyOpenSSL (+cryptography) dependency. The two touch the same _activate_tls, so only one is needed. The trade-offs:

• Dependency: this PR adds none (pure stdlib); feat(tls): use pyOpenSSL for export_keying_material #772 adds pyOpenSSL.
• Real-hardware behaviour: I tested both against real Siemens PLCs — details in a comment on feat(tls): use pyOpenSSL for export_keying_material #772. In short, feat(tls): use pyOpenSSL for export_keying_material #772's pyOpenSSL backend does not restrict the offered TLS 1.3 groups, so real S7-1200/1500 PLCs RST the handshake; this PR keeps the stdlib set_ecdh_curve restriction that they accept.

(No "ctypes" angle here — pyOpenSSL uses cffi, not ctypes; the only real difference is the added dependency.)

Validation

Tested over TLS 1.3 against two real PLCs:

• S7-1200 CPU 1211C, FW V4.6
• S7-1500 soft-controller CPU 1507S F, FW V21.9

On both: oms_secret is now populated (32 bytes) instead of None, and the derived value is byte-for-byte identical to pyOpenSSL's export_keying_material(b"EXPERIMENTAL_OMS", 32) on the same session (cross-checked separately). Anonymous V2 reads still work; falls back to oms_secret=None for non-TLS-1.3 sessions.

Scope / not included

This only unblocks the exporter. The legitimation challenge fetch (GetVarSubStreamed on SERVER_SESSION_REQUEST=303) is still rejected by the 1211C (return_value=0xC318890001E1FFFE), so full password legitimation needs further protocol work — tracked in #718.

Happy to add a unit test for _tls13_exporter (fixed vector) if you'd like.

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