feat(engine): add transient in-memory graphs

[mrkorchun]

What changed

• add Omnigraph::in_memory(schema) for isolated transient embedded graphs
• reuse Lance native shared-memory object store instead of adding another backend implementation
• keep the shared-memory URI private and preserve the existing local/S3 storage API
• add an insert then select integration test

Why

Embedded runtimes need the normal typed graph operations without filesystem hard-link requirements or a durable graph directory. This gives them the same schema, mutation, query, manifest, and branch machinery as durable graphs while keeping lifecycle explicitly process-local.

Validation

• cargo fmt --all -- --check
• git diff --check
• focused lifecycle test added; GitHub CI is the execution gate because the available local Cargo targets were cold and a full Android/Termux rebuild was intentionally not repeated

Known ceiling

Lance retains shared-memory authorities for the process lifetime. Explicit reclamation is deferred until long-lived embedded hosts demonstrate that it is needed.

Greptile Summary

The PR adds isolated, process-local in-memory graphs while preserving the existing graph initialization, manifest, mutation, query, and branch machinery.

• Adds Omnigraph::in_memory(schema_source) using Lance’s native shared-memory URI scheme.
• Extends URI and object-path handling for shared-memory datasets.
• Registers the constructor in the closed public API registry.
• Documents the transient lifecycle and adds an insert/query integration test.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the public API registry now classifies the constructor, and the owning user documentation describes its transient and non-reopenable lifecycle.

Important Files Changed

Filename	Overview
crates/omnigraph/src/db/omnigraph.rs	Adds the documented transient constructor and routes it through the established initialization machinery with an isolated ULID-backed authority.
crates/omnigraph/src/storage.rs	Preserves shared-memory URIs during joining and normalization while leaving the public local/S3 storage-selection API unchanged.
crates/omnigraph-storage/src/lib.rs	Adds shared-memory URI path decoding for the existing in-memory object-store adapter.
crates/omnigraph/src/db/manifest/layout.rs	Keeps shared-memory table locations as native URIs instead of converting them to filesystem URLs.
crates/omnigraph/src/db/manifest/metadata.rs	Converts shared-memory dataset URIs into object-store-relative metadata paths.
crates/omnigraph/tests/forbidden_apis.rs	Correctly classifies in_memory as a bootstrap write surface, resolving the previously reported registry failure.
crates/omnigraph/tests/lifecycle.rs	Covers initialization, typed mutation, and query behavior for a transient graph.
docs/user/concepts/storage.md	Resolves the previously reported documentation gap by describing discoverability, process-local lifetime, non-reopenability, and supported usage.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    Caller[Embedded Rust caller] --> Constructor[Omnigraph::in_memory]
    Constructor --> URI[Private shared-memory URI]
    Constructor --> Init[Shared graph initialization path]
    Init --> Metadata[In-memory schema and recovery metadata]
    Init --> Lance[Lance shared-memory datasets]
    Lance --> Operations[Typed mutation, query, manifest, and branch operations]
    Operations --> Exit[State discarded at process exit]

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_{Reviews (3): Last reviewed commit: "docs(storage): document transient graphs" | Re-trigger Greptile}

Context used:

• Knowledge Base — OmniGraph core storage: manifest, table store, and Lance access