Add PeekNextIndex()

feat(uid): batch allocation (NextN) and in-place pool Init

   Add UID64Pool.NextN(dst) to reserve many identifiers at once into a
   caller-owned buffer: it drains recycled indices, then allocates one
   contiguous run, growing the generation table at most once. ~2x faster
   per id for bulk creation, zero allocation. Refactor grow into a shared
   ensure(need) used by both Next and NextN.

   Replace the NewUID64Pool constructor with an in-place UID64Pool.Init
   method (params indexCap, recycleCap) for consistency with the rest of
   the ecosystem.

   Add package doc.go, runnable Example tests for the pool lifecycle and
   batch path, and update the README pool snippet to Init.

   BREAKING CHANGE: NewUID64Pool is removed; use `var p UID64Pool; p.Init(...)`.

v0.1.1: API Refactoring & CI Integration

Initial release of UID64 generator and pool