Adds 3 hc submission ports (2526/2527/2528) in the single Postfix instance,
each content_filter'd onto a dedicated hc transport (hcout1/2/3) binding the
hc IPs .107/.108/.109 with hc HELO identity (hcmta01-03) and hotter concurrency.
listmonk-hc round-robins the 3 ports.
Discovered + documented the constraint that drove this shape: transport_maps
randmap is owned by the shared trivial-rewrite(8) and is global, so neither a
per-smtpd -o transport_maps nor a FILTER randmap:{...} can scope a separate IP
pool (FILTER parses randmap as a literal transport). content_filter=hcoutN:
(empty nexthop) overrides transport_maps and keeps the real recipient domain.
Verified end-to-end on the server: :2527 -> hcout2 (.108) -> real gmail MX;
trucking transport_maps (.94-.96) untouched. Idempotent, postfix-check gated
with auto-rollback.
