Gate DMARC/SPF/MTA-STS on MX presence to reduce noise

[liquidsec]

Summary

• Adds BadDNS_email_base (in baddns/lib/email_base.py) — shared intermediate base for the email-related modules. Provides has_email_infra() (MX check on target, falling back to the registered/apex domain) and mx_gate_skips().
• DMARC and SPF now skip dispatch when neither the target nor its apex has any MX records. MTA-STS still runs — but the orphaned-TXT and policy-MX-mismatch findings are suppressed without MX, while the dangling mta-sts.* subdomain takeover and policy-MX WHOIS takeover findings keep running unconditionally (they're real takeover vectors regardless of mail flow).
• New CLI flag --disable-mx-gate and matching disable_mx_gate kwarg for opt-out (useful for BBOT and explicit "I want everything" runs).
• get_all_modules() now walks transitive subclasses and filters on the name class attribute, so intermediate bases like BadDNS_email_base aren't returned as modules.
• Existing email tests pass disable_mx_gate=True to preserve their policy-logic focus; new tests cover the gate (skip without MX, run with subdomain MX, run with apex MX, opt-out via flag, MTA-STS finding-by-finding behavior).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.94%. Comparing base (0bef233) to head (19f9831).
⚠️ Report is 3 commits behind head on dev.

Files with missing lines	Patch %	Lines
baddns/lib/email_base.py	97.14%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##               dev     #892      +/-   ##
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- Coverage   100.00%   99.94%   -0.06%     
===========================================
  Files           26       27       +1     
  Lines         1927     1977      +50     
===========================================
+ Hits          1927     1976      +49     
- Misses           0        1       +1     

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