Stabilize SMP priority_order selftest

[jserv]

The SMP build-and-test job hung on selftest 32 (priority_order) past the 120s watcher budget and was killed.

• The make watcher's CHECK_*_TIMEOUT used := so the CI workflow's CHECK_SELFTEST_TIMEOUT=240 env override never applied; the effective budget stayed at the 120s default. Switch to ?= so the override propagates.
• test_priority_order waited for the three pinned worker tasks via a single sleep_ms(50). That wake path arms one callout and depends on a same-priority same-hart re-enqueue to drag the idle thread out of wfi: which is fragile under QEMU TCG SMP timing and has been observed to stall the test indefinitely. Replace the blind sleep with an atomic done-counter polled via sleep_ms(0) yields and a 2s wall-clock deadline, so the test makes progress on its own and fails cleanly if the workers never run.
• dl_replenish_cb only re-enqueued the task when it found it in TD_STATE_DL_THROTTLED, missing the case where the task is already sitting in pcpu_dl_runq[cpu] in TD_STATE_READY (sched_dl_pick_next was skipping it because dl_throttled was set). Without that poke the owning hart can stay parked in wfi after the throttle flag clears. Set need_resched on the task's CPU (and cross-hart IPI) when we observe READY at replenish time.

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Summary by cubic

Stabilizes the SMP priority_order selftest and fixes a deadline scheduler wakeup gap to prevent CI hangs and ensure replenished tasks run promptly. Also makes CI selftest timeout overrides work.

• Bug Fixes
  • Makefile: switch CHECK_TIMEOUT and CHECK_SELFTEST_TIMEOUT to ?= so CI env overrides apply.
  • Selftest: replace fixed sleep_ms(50) with an atomic done counter polled via sleep_ms(0) yields and a 2s deadline to avoid SMP stalls and fail cleanly if workers never run.
  • Scheduler: in dl_replenish_cb, when a task is READY, set need_resched on its CPU (and send IPI if needed) so it’s picked after replenishment; previously only handled DL_THROTTLED.

^{Written for commit b87ac2b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files