Replace prealloc-passing with Release()-based pooling for term postings

[capemox]

Summary

Part of the bitpack-simd effort (zapx blevesearch/zapx#447, scorch_segment_api companion PR): replaces the prealloc-passing pattern in TermFieldReader() and prepareField() with pooled reuse.

• rv.postings[i]/rv.iterators[i] used to be handed back in as a same-shaped "prealloc" hint on every lookup, but a recycled reader's shape is frequently wrong (a mix of 1-hit and general-encoded terms landing on the same field), so the hint was often discarded and allocated over anyway.
• Both call sites now call releasePostings() -- a type assertion against the new, optional segment.Releasable capability -- on a value they're done with, and fetch a fresh one with prealloc left nil, letting the segment implementation's own pool decide what to hand back.
• segment.Releasable is an opt-in capability, the same idiom as the existing segment.OptimizablePostingsIterator: values that don't implement it (e.g. the unadorned/synthetic iterators built by query optimization) make releasePostings() a no-op, unchanged from before.
• go.mod bumped to the scorch_segment_api bitpack-simd branch, which adds Releasable plus MergeablePostingsList/MergeablePostingsIterator as optional interfaces (not required additions to PostingsList/PostingsIterator, so zapx v11-v17 keep compiling unchanged).

Test plan

• go build ./..., go vet ./... clean
• go test ./... clean
• go test -race ./index/scorch/... clean

[coveralls]

Coverage is 51.716% — bitpack-simd into next. No base build found for next.

[capemox]

Putting this off for a future release -- reverting the split/pooling approach in zapx. Closing rather than merging.