feat(miner-portfolio): define PortfolioQueueManager types + pure bucketing primitives

[JSONbored]

The miner's local queue needs a deterministic, pure data-structure layer before any enforcement logic can sit on top of it. This issue is foundation-only: types + pure functions for a bucketed, WIP-capped, cross-repo-diversified queue — no I/O, no SQLite, no wiring into a real run loop yet.

Mirror the shape of src/services/plan-dag.ts (pure PlanDag/PlanStep types + buildPlanDag/validatePlanDag/nextReadySteps free functions, no class, no side effects) for a new PortfolioQueue concept: items bucketed by repo, a per-bucket and global WIP cap, and a pure nextEligibleItems(queue, caps) selector that respects both caps and prefers under-represented repos (cross-repo diversification) using the same kind of stable-sort/min-based tie-breaking src/signals/duplicate-winner.ts already uses for cluster elections.

Deliverables

• New packages/gittensory-engine/src/portfolio/queue.ts (or equivalent path under the new shared engine package) exporting PortfolioQueueItem, PortfolioQueue, PortfolioCaps types
• Pure enqueueItem(queue, item), dequeueItem(queue, itemId), nextEligibleItems(queue, caps) functions — no Date.now(), no randomness, deterministic given the same input
• Cross-repo diversification: nextEligibleItems never returns two consecutive items from the same repo bucket while another repo bucket has eligible items waiting
• Unit tests covering: empty queue, single-repo queue at WIP cap, multi-repo diversification ordering, and a full/blocked-queue edge case
• JSDoc header explaining this is pure bookkeeping — no enforcement decision, no write action — consistent with how plan-dag.ts documents itself as stateless

References

• src/services/plan-dag.ts (134 lines) — the pure DAG/type pattern to mirror
• src/signals/duplicate-winner.ts (60 lines) — the min-based deterministic election pattern to reuse for tie-breaking
• packages/gittensory-engine/ (new shared engine package per the miner architecture skeleton)

[linear-code]

JSO-2423