[AdaAibaby]

Next step toward #3763 (raising sandboxes-per-node), building on the observability in #3766. Flag-gated, strict no-op by default.

The gap (what's already done, and what isn't)

Free-page hinting already returns a guest's freed pages to the hugepage pool: the guest reports free pages, Firecracker calls discard_range, and for the shared/hugetlbfs memfd backing that is madvise(MADV_REMOVE) — which punches the backing file and frees the physical hugepages. (Verified by disassembling the deployed v1.14-0.2.0 binary: discard_range uses MADV_DONTNEED only for anonymous mappings and MADV_REMOVE for the shared memfd; a quick kernel experiment confirms MADV_REMOVE/PUNCH_HOLE return hugepages to the pool while MADV_DONTNEED does not.)

What is not adaptive is when hinting runs. The periodic loop ticks on a fixed interval, blind to how full the node's hugepage pool is. So between a guest freeing memory and the next tick (tens of seconds at the default interval), those pages stay charged to the pool. On a densely packed node that reclaim-lag window is exactly what makes placement refuse a node that is about to have room — the pool reads full of pages the guests have already freed but not yet hinted back.

Change

• PeriodicHintingConfig gains PressureInterval and PressureThresholdPct. When the node hugepage pool is at/above the threshold, the loop waits the shorter PressureInterval instead of Interval, so freed pages return to the pool sooner and the reclaim-lag window shrinks. Below threshold, or either knob unset → the fixed Interval (unchanged default).
• effectiveHintInterval — pure, fully tested: base interval unless pressure ≥ threshold AND a shorter PressureInterval is set; never slows hinting down; unknown pressure (-1) fails safe to the base interval.
• Node pool pressure read from /proc/meminfo (HugePages_Total/Free) with a 1-second process-level cache, so the per-sandbox loops don't each read meminfo every tick — one read/second serves every sandbox on the node. hugePageUsagePct is also pure/tested.

Safety / scope

• The only behavioural change is the inter-tick wait under pool pressure. It does not touch the UFFD serve path, the discard/punch path, or any of the latch / silent-guest / unresponsive-retry invariants in the hinting loop.
• PressureInterval = 0 (default) → strict no-op.
• go build/go vet clean; pure-function tests (interval selection, usage %, TTL caching) pass; existing hinting/periodic suites pass.

Relationship to #3766

#3766's reclaimable_estimate gauge quantifies the exact reclaim-lag window this PR shrinks, and is how an operator picks PressureThresholdPct / PressureInterval. Measure first (#3766), then act (this).

Refs #3763.