Add redirect helpers to ha.c/ha.h so a standby controller never accepts
join requests as if it were the primary:
- ha_should_redirect(): returns 1 when role is STANDBY and primary_host/port
are known — caller should send E_XCE_REDIRECT.
- ha_no_primary_known(): returns 1 when role is STANDBY but primary address
is not yet known — caller should send E_XCE_NOMASTER (retry later).
- Add E_XCE_NOMASTER and E_XCE_REDIRECT to errnox.h.
- Wire both guards into handle_join_() in join.c (before the role switch)
so all join types (worker, scheduler, apiserver, client) are protected.
- Full redirect response sends status=-E_XCE_REDIRECT with nodata body;
the primary host:port are logged so operators can trace misrouted clients.
Full Cap'n Proto RedirectInfo response body is deferred to a follow-up
(schema union would require regenerating capnp bindings).
5 new TDD tests — 110 total, 0 failures.
