Persist schema view layout selection

[mjuarros]

Description

Persist the selected Schema View layout algorithm as an application-level preference so reopening the Schema Explorer restores the user's last choice. The change also centralizes valid layout names and safely normalizes legacy or unrecognized persisted values to F_COSE.

Per-connection persistence is intentionally excluded because there is no evidence that different connections need different Schema View defaults, and introducing keyed state would add lifecycle complexity without a demonstrated requirement.

Current behavior (No layout persistence)

New behavior (persisted layout for the session)

How to read

1. packages/graph-explorer/src/core/graphLayout.ts — canonical layout names, type, default, and runtime validation
2. packages/graph-explorer/src/core/StateProvider/schemaViewLayoutDefaults.ts — persisted preference and backward-compatible normalization
3. packages/graph-explorer/src/modules/SchemaGraph/schemaGraphLayout.ts — focused writable atom consumed by the Schema Graph and toolbar

Validation

• pnpm checks
• pnpm test (220 files, 2677 tests)
• Formal Standards + Spec review
• Independent Devin code review

Related Issues

• Closes Save layout algorithm choice in schema explorer #1430
• Related to Save layout choice to saved graph file & previous session #890
• Related to Graph Layout Enhancements #2017

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.