Add optional MergeablePostingsList/Iterator and Releasable interfaces

[capemox]

Summary

Part of the bitpack-simd effort (zapx blevesearch/zapx#447, bleve companion PR): adds three optional capability interfaces, following the existing OptimizablePostingsIterator pattern rather than changing the base PostingsList/PostingsIterator contracts.

• MergeablePostingsList (RawDocFreq() uint64, HasLocs() bool) and MergeablePostingsIterator (NextWithLocBytes(...)) let a segment-merge path depend on published interfaces instead of a package-private bridge.
• Releasable (Release()) lets a PostingsList/PostingsIterator value opt into pooled reuse, replacing prealloc-passing as the primary reuse mechanism for implementations that choose to support it.

These are additive and optional by design: bleve unconditionally registers zapx v11-v17 as segment plugins, none of which implement these methods, so adding them as required methods on the base interfaces would have broken bleve's build against six already-released modules. Checking via type assertion (the same idiom bleve's optimize.go already uses for OptimizablePostingsIterator) avoids that entirely.

Test plan

• go build ./..., go vet ./... clean

[capemox]

Putting this off for a future release -- the zapx side (interface-only merge + pooling) is being reverted to the single PostingsList/PostingsIterator implementation. Closing rather than merging.