feat(io): export deterministic graph data subsets with referential closure

[DecisionNerd]

Problem

Component-level selection cannot express a specific graph or tabular data subset. Ad hoc filtering risks nondeterministic results, dangling edges/references, ontology mismatch, leaked properties, and packages whose claimed identity cannot be reproduced.

Objective

Export deterministic graph/data subset packages from one pinned generation with explicit referential-closure, schema/ontology, provenance, and privacy semantics.

Requirements

• Define typed selectors over stable UUIDs, named datasets/participants, and supported deterministic public query/projection results. Never expose runtime catalog IDs as portable identities.
• Define at least induced-node and edge-with-endpoints closure modes; reject dangling endpoints or references unless the selected package class explicitly models them.
• Pin one source generation/checkpoint and record the canonical selector, source fingerprint, closure mode, ontology/schema requirements, property projection/redaction, counts, and result fingerprint.
• Require deterministic ordering and reject selectors whose semantics/order cannot be reproduced under the portable-v2 contract.
• Stream selection and package writing with bounded memory, backpressure, cancellation, progress, finite output/count limits, and no full subgraph materialization requirement.
• Preview exact selector semantics, estimated counts where safe, dependency closure, projected/redacted fields, and authorization/privacy implications before writing.
• Verify that every emitted edge endpoint/reference and every declared ontology/schema dependency is present or explicitly externalized by a supported contract.
• Keep consume/import semantics explicit; creating a new subset project is allowed by the format, but merging into an existing graph is not introduced here.

Acceptance criteria

• Repeating a supported selector against the same pinned source produces the same selection fingerprint, counts, ordered content, and package digest.
• Induced-node and edge-with-endpoints fixtures prove referential closure and stable UUID preservation.
• Property projection/redaction tests prove excluded fields and unrelated rows do not leak through payloads, manifests, errors, or provenance.
• Ambiguous, nondeterministic, dangling, unsupported, or resource-exceeding selectors fail before a final package is published.
• Large subset export remains bounded by execution/writer configuration rather than total selected bytes.
• The resulting subset verifies through feat(io): verify expanded and bundled portable projects #784 and can be published/pulled without changing identity.

BDD completion scenarios

Scenario: Export an induced subgraph

Given stable selected node UUIDs in a pinned generation
When induced-node closure is exported
Then the package contains exactly those nodes, permitted properties, and edges whose endpoints are both selected
And its receipt records deterministic counts, ontology/schema dependencies, and selection fingerprint.

Scenario: Export selected edges safely

Given stable selected edge UUIDs
When edge-with-endpoints closure is exported
Then both endpoint nodes are included with the declared property projection
And no dangling edge or silent extra neighborhood is emitted.

Scenario: Reject nondeterministic selection

Given an unordered or time-dependent selector without a canonical snapshot/order contract
When export is planned
Then it fails with a typed determinism error
And no final package is published.

Observability and security

Report aggregate source/scanned/selected/rejected/redacted counts, phase timing, spill/progress, and failure codes. Authorization remains caller-owned; selectors, property values, UUID inventories, and graph content must not enter logs.

Testing and documentation

Cover closure modes, isolated nodes, self-loops, multiedges, missing endpoints, ontology constraints, property redaction, deterministic reruns, cancellation/spill/resource limits, corruption verification, and clean subset-project reopen. Document reproducible subset recipes and privacy review.

Non-goals

Graph merge/upsert, arbitrary nondeterministic query export, inference, anonymization guarantees, policy-engine design, or foreign execution engines.

Related issues

Portable-v2 epic: #740. Component selection prerequisite: #785. Streaming result export remains #743.