feat: ha_should_redirect + join.c HA redirect/nomaster guard (P1-6)

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.

src/controller/meson.build

@@ -85,6 +85,7 @@ controller_unit_test_src = [
   'testsuite/test_e2e_capnp.c',
   'testsuite/test_ha_role.c',
   'testsuite/test_ha_cluster.c',
+  'testsuite/test_ha_redirect.c',
   'persist.c',
   'ha.c',
 ]