[WIP] Move in-memory providers to diskann-inmem crate - blocked by orphan rules

[Copilot]

Attempts to extract in-memory providers from diskann-providers into a standalone diskann-inmem crate. The refactoring is blocked by Rust's orphan rules - the inmem providers implement diskann-providers traits for diskann-providers types, which is only permitted within the same crate.

Changes Made

• Created diskann-inmem crate: New workspace member with provider implementations moved from diskann-providers/src/model/graph/provider/async_/inmem/
• Updated import paths: All moved files now import from diskann_providers:: instead of crate::
• Migrated helper APIs: Moved diskann_async module and storage implementations to new crate
• Updated consumers: diskann-benchmark and diskann-disk now depend on and import from diskann-inmem
• Relocated SQError: Moved to diskann-providers/src/storage/sq_storage.rs to avoid circular dependencies

Orphan Rule Violations

Multiple trait implementations violate Rust's orphan rules:

// In diskann-inmem, attempting to implement diskann-providers trait
// for diskann-providers type - not allowed
impl CreateVectorStore for FixedChunkPQTable { ... }
impl<T> SetElementHelper<T> for FullPrecisionStore<T> { ... }

The code compiled when in diskann-providers because trait, implementation, and types were co-located.

Resolution Options

1. Newtype wrappers: Wrap external types in diskann-inmem-defined newtypes
2. Split trait/impl: Keep implementations in diskann-providers, move data structures only (creates circular deps)
3. Revert: Keep inmem organized as submodule within diskann-providers
4. Trait redesign: Define inmem-specific traits in diskann-inmem (larger refactor)

Current state: crate structure exists but doesn't compile. Requires architectural decision before proceeding.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Move in-mem provider from inside diskann-providers to its own crate</issue_title>
<issue_description>

In-memory providers are in diskann-providers/src/model/graph/provider/async_/inmem. Move them up to diskann-inmem crate</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Move in-mem provider from inside diskann-providers to its own crate #712

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[harsha-simhadri]

@copilot List all instances of orphan rule violations and lets work on resolving case by case