Now that Imperva is fully solved (native-TLS no-browser bypass for light usage,
browser-solve reese84 for heavy usage), bring all docs in line:

- README.md: new "Imperva / Incapsula (no-browser bypass)" section, updated the
  challenge-escalation steps, added _native_tls.py to the file layout.
- llms.txt: corrected the section heading (browserless only at light load).
- docs/ref-imperva.md: documented the rotation-skip (both paths go straight to
  the browser; rotation never helps a TLS-stack challenge) and proxy gating.
- docs/ref-sec-fetch.md: connected the "omit Sec-Fetch to look non-browser"
  insight to the realized native-TLS implementation.
- docs/site-list.md: api2.realtor.ca entry now covers light (12/12 no-browser)
  + heavy (18/18 browser-solve) and the http.client transport.
- todo-realtorsolve.md: heavy usage is solved, no longer an open caveat.