fix(component-failover-adoption): Phase 6 -- close apiserver's idle-period rejoin gap (bug-1548/bug-1549)

XCECLI_FF_CALL (backing xcecli_heartbeat) can silently rotate+reconnect
the transport entirely inside a single call and still return success, so
heartbeat.go's failure-to-success transition detector could never observe
a reconnect that completes within one heartbeat tick -- the common case,
since elections finish in single-digit seconds against a default 30s
interval. apiserver's application-level rejoin never fired on a real
idle-period failover, and job pushes silently stopped reaching it.

Adds a monotonic reconnect_gen counter to endpoint_t (netio.h), bumped by
xce_ctx_reconnect() (xcectx.c) on every successful reconnect regardless of
trigger path. heartbeat.go now compares this generation every tick instead
of tracking success/failure transitions. Re-validated live against task
5.2's exact scenario (apiserver alone, zero write traffic): rejoin fired
22s after a primary kill, 12s after election.

Reopens and closes out component-failover-adoption's own NO-GO -> GO
validation cycle from 2026-08-15.

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

src/lib/meson.build

@@ -178,6 +178,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_libxce_retry.c',
                    'testsuite/test_netio_on_connected.c',
                    'testsuite/test_netio_on_fini.c',
+                   'testsuite/test_netio_reconnect_gen.c',
                    'testsuite/test_netio_arm_failure.c',
                    'testsuite/test_netio_close_propagation.c',
                    'testsuite/test_netio_bounded_connect.c',