Lease-service side: maxActiveHoldMs reactive config + shouldYield() method — #14144 sub

The lease-service half of the #14144 heavy-maintenance lease-fairness epic. (Grace owns the consumer half, #14186 — the kbSync embedViaShadowSwap between-batch yield-point.)

Scope

HeavyMaintenanceLeaseService exposes the cooperative-yield decision so a long heavy-task holder can poll it between resumable checkpoints:

• a reactive maxActiveHoldMs_ config reading AiConfig.orchestrator.heavyMaintenance.maxActiveHoldMs at the use-site (SSOT discipline);
• a thin shouldYield(lease, {now, maxActiveHoldMs}) method over the already-built shouldYieldHeavyMaintenanceLease primitive;
• the leaf default set to 30min (the fairness Decision Record converged with @neo-opus-grace: independent of staleAfterMs but smaller, so a live holder yields before stale-reclaim; a soft knob, tunable on churn);
• falsy ⇒ never yields (byte-identical back-compat — #14186's absent/0-leaf path).

Acceptance Criteria

• HeavyMaintenanceLeaseService.shouldYield(lease) injects the reactive maxActiveHoldMs into the primitive (per-call override wins).
• maxActiveHoldMs leaf defaults to the 30min DR; env-overridable; falsy ⇒ never yields.
• Unit test covers the service method (injection, override, fail-safe).

Delivered by the PR linked below. The #14144 epic closes once this and #14186 (the consumer) land. Refs #14144 (parent epic), #14186 (consumer sub), #14039 (v13.1).

Contract Ledger

This sub modifies consumed surfaces (pull-request-workflow guide §5.4) — the matrix (mirrored from the delivering PR #14261):

Existing acquire / inspect / release / withLease + the re-exported pure primitives are unchanged.

Authored by Ada (@neo-opus-ada · Claude Opus 4.8, Claude Code).