feat: deadline helpers (#852), multi-invoice payment aggregator (#853), options-only tenant pool (#846), property tests (#845) - #898

Summary

Closes four open issues in one PR:

• Add deadline engine — helpers for computing and validating invoice deadlines #852 — deadline engine helpers. New src/deadline.ts module with bigint-based helpers that match the on-chain u64 representation: deadlineFromDays (rounded up so any positive duration is strictly in the future), deadlineFromDate, isDeadlineValid (1-hour minimum lead time), timeUntilDeadline (returns { days, hours, minutes, seconds, expired }, all zeros once expired) and formatDeadline (locale-aware, rendered in UTC for deterministic output). Exported from the package root and documented in the README. The legacy number-returning deadlineFromDays on the @stellar-split/sdk/utils entry point is left untouched and its README row points to the new helper.
• Implement payment aggregator — compute optimal payment split across multiple invoices #853 — payment aggregator. New src/paymentAllocation.ts module implementing aggregatePayments(budget, invoiceIds, strategy, options?) with all three strategies:
  • equal — even split, surplus redistributed;
  • proportional — weighted by remaining amount, or by fraction of target still unfunded when targets are supplied (so invoices furthest from their target receive the most);
  • custom — caller weights that are validated to sum to 100 (length, non-negativity and finiteness are also validated).
    Every allocation is capped at the invoice's remaining amount so overpayment is impossible, and rounding dust is redistributed deterministically with the largest-remainder/water-filling algorithm, guaranteeing the allocations always sum to min(budget, total remaining). Empty invoice lists return []; duplicate IDs, negative budgets, unknown strategies and invalid weights throw ValidationError. Remaining amounts are resolved from an explicit map/record, a fetchRemaining function, an invoiceSource (any object with getInvoice(id)), or a fetcher registered via registerInvoiceRemainingFetcher — with remainingForInvoice and createInvoiceRemainingFetcher helpers for the on-chain recipients - funded derivation.
• Implement multi-tenant client pool with TTL eviction and health checks #846 — multi-tenant client pool. The pool already implemented LRU eviction, TTL expiry, background health checks and stats(), but it could only be created as new MultiTenantClient(factory, options) while the documented API is new MultiTenantClient({ maxClients, ttlMs, healthCheckIntervalMs }). The constructor now accepts either form: factory + options (unchanged, backwards compatible) or options alone, in which case getClient(tenantId, config) supplies the tenant config. A missing factory and config now raises a descriptive ValidationError instead of an opaque TypeError.
• Add property-based testing suite using fast-check #845 — property-based testing. fast-check was already a dev dependency with five property files; this PR extends the suite to the new modules and wires the suites into npm test so the criteria are actually exercised by CI: new test/property-payment-allocation.test.ts (7 properties, 500 examples each: budget never exceeded, no overpayment per invoice, min(budget, total remaining) fully distributed, input order preserved, percentages in [0, 100], valid weights accepted, invalid weights rejected) and 4 new 500-example properties in test/property-deadline.test.ts for the bigint helpers (future timestamps for any n > 0, the 1-hour rule, non-negative countdown units, exact duration reconstruction).

Approach

• New, self-contained modules (src/deadline.ts, src/paymentAllocation.ts) following the existing conventions: JSDoc on every export, ValidationError from src/errors.js for input validation, registered-fetcher pattern (registerInvoiceRemainingFetcher) like registerInvoiceFetcher/registerChannelStateFetcher, and root re-exports added to src/index.ts with the #nnn — section banners used by the rest of the file.
• bigint arithmetic end-to-end (no floats) for allocations; the water-filling allocator caps shares at each invoice's remaining amount and redistributes the surplus over remaining passes, then resolves sub-stroop rounding with the largest-remainder method.
• src/multiTenant.ts keeps its existing behaviour and only widens the constructor signature (factory | options) plus the guard in getClient, so all existing call sites and tests keep working.

How it was tested

ESLint is not configured in this repository (npm run lint maps to tsc --noEmit), so that check is covered by the type-check run above.

closes #852
closes #853
closes #846
closes #845