Replace monolithic add_worker with three focused functions:
- apply_join_state (static under production, exported under XCE_TESTING):
shared status/resource update called by both create and rejoin paths;
clears XCE_WORKER_UNAVAIL, applies hconfig overrides, updates resources
- init_worker: first-time join path via makeWorker + apply_join_state
- rejoin_worker: reconnect path with explicit admin-bit preservation
(XCE_WORKER_ADMIN_MASK = LOCKEDU|CLOSED|HIDDEN|CLOSED_BY_ADMIN)
Also makes find_matched_hostconfig non-static so join.c can call it
via the extern declaration already in worker.h.
Removes add_worker (no longer declared in worker.h, no callers).
All 7 tests in test_worker_join pass:
apply_join_state (clears UNAVAIL, hosttype override, resource values),
init_worker (creates entry, returns correct idx),
rejoin_worker (updates attrs/ep, preserves admin bits).
