feat: public batch merge and snapshot install

[St4NNi]

Lets an application replicate craqle batches over its own transport and seed replicas from graph snapshots, so nodes that cannot join the irokle topics (Aruna user devices) still converge with the OR-Set semantics of the store.

Changes

• CraqleNode::merge_batch: merges a batch authored elsewhere with the semantics of the irokle apply path (tombstone check, dot dedupe under the graph write guard, OR-Set apply in op order, diagnostics refresh, post-commit SHACL settlement that never rejects); idempotent by dot.
• Batch::from_changes: one public builder for the op mapping; the irokle path now uses it too.
• CraqleNode::install_graph_snapshot: state-based OR-Set join with causal context (a dot the local clock covers but the local quad lacks is a known removal), element-wise clock max, idempotent; a lagging replica's snapshot never resurrects a removed quad.
• plan_rocrate_document_checked, plan_patch_data, plan_patch_contextual: change sets without applying, same validation as the applying variants.
• graph_snapshot orders quads and dots deterministically and is documented as a sync mechanism; MergeResult and CrossGraphChange are public.
• Two nightly clippy findings fixed (as_chunks in the export cursor decoder, the deprecated fetch_update allowed under the current MSRV).
• Tests: tests/merge_api.rs (merge equality, concurrent add and remove convergence in either order, snapshot idempotence, stale snapshot ignored, planner purity).

References

• arunaengine/aruna user-nodes and desktop arc: OR-Set metadata graphs and offline devices.