feat(arsenal): subdomain takeover detection built-in (#134)

* feat(arsenal): add subdomain-takeover classifier + fingerprint table

Pure, I/O-free detection logic split into src/arsenal/takeover.ts so it is fully
unit-testable without DNS or network:
- TAKEOVER_FINGERPRINTS: curated CNAME + unclaimed-resource-body signatures for 15
  high-frequency services (S3, GitHub Pages, Heroku, Azure, Fastly, Shopify, Netlify, ...)
- classifySubdomainTakeover(): conservative verdict (confirmed / potential / none) driven by
  a dangling CNAME and/or a service body fingerprint; no cross-service false matches
- renderTakeoverReport(): human-readable output block

* feat(arsenal): register subdomain_takeover_check built-in + bump tool count 108->109

- New keyless, self-contained built-in tool 'subdomain_takeover_check' (recon): resolves the
  CNAME, checks whether the target still resolves (dangling), fetches the live body via the
  scope-gated targetFetch, and classifies via classifySubdomainTakeover. Emits a real finding.
- Added to the Recon operator's default toolkit so the swarm can reach it (operator-toolkits
  coverage invariant).
- Bumped the advertised arsenal size 108 -> 109 in lockstep across the count-honesty test,
  README, and verify-claims (73 adapters + 36 built-in).

* test(arsenal): cover subdomain_takeover_check (classifier matrix + mocked DNS/fetch e2e)

- classifySubdomainTakeover decision matrix: confirmed via body fingerprint, confirmed via
  dangling nxdomain-prone CNAME, potential (known service, no confirmation), potential
  (dangling unknown), none (no CNAME / resolves clean), and no cross-service false matching
- renderTakeoverReport output shape
- end-to-end handler test with DNS + fetch mocked, driving the confirmed S3 path