feat(cpu-alloc): run the lvstore poller group on the nvmf poller cores

[wmousa]

Summary

The lvol poller group was pinned to a single core: the JC singleton's below 32 vCPU, a dedicated one above. One busy core then throttled every lvstore and transfer-hub bdev at once. The 2026-09-29 test build (26f57dd72, MERGE_LVOL_POLLER_WITH_POLLER_CORES, since reverted) checked that by handing the nvmf pollers and the lvol poller group one shared core set at mask-build time.

This makes it the allocation itself, no flag:

• calculate_core_allocations() derives lvol_poller_core from poller_cpu_cores in every tier (utils._lvol_poller_cores), so lvol_poller_mask, still the single source of truth that add-node and restart send to bdev_lvol_create_poller_group, is the multi-core poller mask.
• No core is reserved for it. Below 32 vCPU the layout is exactly what it was. At 32 and above the core the old layout dedicated to the lvol poller returns to the distrib/alceml/poller pool, so the base roles take 3 cores at every size; the 24-core distrib step therefore lands at 37 vCPU instead of 38.
• The JC singleton core is the fallback only when a layout has no poller core at all (a 2-vCPU host, or a device count that clips the remainder to nothing).
• Persisted nodes keep their stored masks (restart does not re-derive the role allocation), so the placement applies to nodes added or configured after the upgrade.

Tests

• tests/unit/test_lvol_poller_on_poller_cores.py replaces test_lvol_poller_jc_colocation.py: every tier shares the poller set, no core reserved at ≥32, the multi-core mask, the JC fallback, the tiny tiers.
• test_distrib_priority_over_alceml.py: the 5-vCPU layout, the exclusivity check and the distrib-count table updated for the 3-core base.
• ruff and mypy clean on the changed modules; the unit suite's remaining failures are the pre-existing ones on main (test_spdk_proxy_unit, test_lblk_persistent_names), identical with the change stashed.

Not yet exercised on a cluster; the vm12 cluster is mid-test on the current images.

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