Pluggable index storage for build-while-reading

[vsazhenyuk-softheme]

Problem

The index could only be persisted to a single seekable file, and the access-point table was written only on finish(). That made it impossible to use an index while it was still being built — a separate decoder had nothing to
read until the builder completed — and it hard-wired the index layout to a local file, ruling out other backends or concurrent readers.

Solution

Introduced an IndexStorage trait that abstracts where access points and their 32 KB windows live, alongside new store-backed types (GzStoreIndexBuilder / GzStoreDecoder, plus Deflate* / Zlib* variants). The
decompression core was extracted into a shared Engine, so the existing file-based path is reused verbatim and its on-disk format is unchanged.

This enables:

• Read while building — each access point is handed to append as it's produced, so a decoder sharing the store can seek immediately, with no finish() required.
• Any backend — windows/points can live in a DB, object store, shared memory, etc.
• Concurrency — all trait methods take &self, so one cloneable handle feeds a builder and a decoder on different threads.

Window encoding is selectable via WindowFormat (Raw or Deflate) and is self-describing on read. Backend errors surface through a new Error::Callback variant.

[Copilot]

Pull request overview

Adds a pluggable-storage variant of the index builder/decoder so access points + 32KB windows can be persisted in arbitrary backends (and consumed concurrently while the index is still being built), reusing the same decompression/seek core as the file-backed implementation.

Changes:

• Introduces IndexStorage, Window/WindowFormat, and store-backed *StoreIndexBuilder / *StoreDecoder public APIs.
• Refactors the shared decompression/seek logic into a storage-agnostic Engine + SeekPoint layer used by both file-backed and store-backed paths.
• Adds integration tests for store-backed indexing/decoding and strengthens CI (fmt/clippy/rustdoc warnings as errors) plus a manual release workflow.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/store.rs	Implements the store-backed builder/decoder and the IndexStorage contract.
src/base.rs	Refactors shared decompression/seek code into Engine/SeekPoint and adapts file-backed types.
src/lib.rs	Exposes the new store-backed public API surface via a macro-generated interface.
tests/store.rs	Adds integration tests covering store-backed indexing, concurrent read-while-build, and tar.gz random access.
README.md	Documents the new pluggable storage API and usage.
.github/workflows/rust.yml	Expands CI to include fmt/clippy/docs checks.
.github/workflows/release.yml	Adds a manually-invoked release workflow.

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