FIRMS.md: add verified website column (135/137 firms)

Chunked lookup (10 agents) found + disambiguated each firm's official
homepage — e.g. Millennium->mlp.com, GTS->gtsx.com, Radix->radixtrading.co
(not the crypto radix.trading), Cantab now under gam.com. Two blanks:
Eladian (defunct) and Continuum (no public site).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: whiteskylondon