chore: baseline snapshot — prior-session work + ha-consistency-invariants spec family

Pre-existing uncommitted work: ha-repl-hardening payload data plane (7.x),
heartbeat liveness fix (Req 1.3), connection-reuse ASAN fixes, doc cleanup.
New: ha-consistency-invariants umbrella (requirements/design/research) and
its four implementation sub-specs (specs generated via batch, cross-spec
review applied), steering roadmap + product.md Raft+CP decision.

src/lib/delay.c

@@ -1,5 +1,4 @@
 #include <errno.h>
-#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>