k_work: missing upstream APIs (K_WORK_INIT, K_WORK_DELAYABLE_DEFINE, busy_get, drain/stop, queue_start signature, system-WQ Kconfig)

[swoisz]

Umbrella for small k_work API-surface gaps vs upstream; each is independently landable:

• K_WORK_INIT(handler) — struct-literal static initializer (distinct from K_WORK_DEFINE, which declares a variable). Needed for embedding k_work inside other static structs.
• K_WORK_DELAYABLE_DEFINE(name, handler) — exists but is NOT ready-to-use (documented BEHAVIORAL DELTA: the embedded k_timer needs runtime k_work_init_delayable() because esp_timer requires esp_timer_create()). Revisit whether the linux/HW timer backends allow a true static form.
• k_work_busy_get(work) — flag bitmask (PENDING/QUEUED/RUNNING/CANCELING); trivial wrapper over the existing flags field.
• k_work_queue_drain / k_work_queue_stop — lifecycle APIs; _drain becomes a walk-under-lock after the dlist rewrite (k_work: rewrite queue backend from FreeRTOS xQueue to sys_dlist + k_sem #24); _stop needed for test teardown. _unplug deferred, no consumer.
• k_work_queue_start() signature — today (queue, name, stack_size, prio); upstream is (queue, stack, stack_size, prio, const struct k_work_queue_config *cfg) with cfg carrying name/no_yield/essential. Add the config struct; keep the current signature as a deprecated shim if consumers exist.
• System workqueue Kconfig — prio=5 and stack size are hardcoded in k_work.c. Add CONFIG_SYSTEM_WORKQUEUE_PRIORITY / CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE mirroring upstream names and defaults.

[swoisz]

Scope trim after auditing main: most of this umbrella already exists — K_WORK_INIT ✓, k_work_busy_get ✓, k_work_queue_drain ✓, cfg-struct k_work_queue_start ✓, CONFIG_SYSTEM_WORKQUEUE_PRIORITY/_STACK_SIZE/_NO_YIELD ✓. Still open: k_work_queue_stop (test teardown), and making K_WORK_DELAYABLE_DEFINE truly static-initializable (currently requires runtime k_work_init_delayable() for the embedded timer; the documented BEHAVIORAL DELTA). This issue now tracks just those two.