feat(pipeline): add retry logic, perClass conversion, and tuneable concurrency

[ddeboer]

Summary

Improves pipeline resilience against cold-cache 504 timeouts on external SPARQL endpoints.

• Retry transient HTTP errors: SparqlConstructExecutor now retries on 502/503/504 using p-retry (configurable, default 3 retries)
• Convert classPropertySubjects/classPropertyObjects to perClass: these stages previously ran monolithic queries across all classes, making them vulnerable to timeouts. Now they use per-class execution with inner subqueries compatible with injectValues()
• Tuneable concurrency: new VoidStageOptions interface exposes batchSize and maxConcurrency on all VoID stage factory functions
• voidStages() convenience function: returns all stages in recommended order, optimised for cache warming — classPartitions() runs before per-class stages so the ?s a ?class pattern is cached. Replaces 16-item Promise.all() boilerplate in consumers

Changes

@lde/pipeline

• Add p-retry dependency
• Add retries option to SparqlConstructExecutorOptions (default 3)
• Wrap fetchTriples() in pRetry(), only retrying on transient HTTP errors (502/503/504)
• Add isTransientHttpError() helper parsing the fetch-sparql-endpoint error format
• Three new tests: retry succeeds, non-transient not retried, retries exhausted

@lde/pipeline-void

• Add VoidStageOptions and VoidStagesOptions interfaces
• All stage factories accept optional VoidStageOptions
• classPropertySubjects() and classPropertyObjects() now use selection: 'perClass'
• SPARQL queries rewritten: ?type → ?class, added inner subqueries for injectValues() compatibility
• New voidStages() function with cache-warming stage ordering
• README updated: document voidStages() with options table and usage example, move classPropertySubjects/classPropertyObjects to per-class table, fix query file link paths