feat: integrate sparsifier into pi.ruv.io brain server (ADR-116)

[ruvnet]

Summary

• Integrates ruvector-sparsifier into the brain server's KnowledgeGraph
• Compressed shadow graph maintains spectral properties with O(n log n) edges instead of O(n²)
• All existing data paths unchanged — sparsifier is additive only, no data loss risk

Changes

• Cargo.toml: Add ruvector-sparsifier dependency
• graph.rs: Add AdaptiveGeoSpar field, incremental updates in add_memory/remove_memory, rebuild_sparsifier(), sparsifier_stats(), sparsifier_audit()
• types.rs: Add sparsifier_compression and sparsifier_edges to StatusResponse
• routes.rs: Build sparsifier on startup hydration and rebuild_graph, expose stats in status endpoint

Safety

• Full graph preserved for exact lookups and neighbor queries
• Sparsifier is Option<AdaptiveGeoSpar> — gracefully absent if build fails
• No existing behavior modified — only additive instrumentation
• 141/143 tests pass (2 pre-existing pipeline test failures unrelated)

Test plan

• cargo check passes
• cargo build succeeds
• 141 tests pass (2 pre-existing failures in pipeline::tests)
• Deploy to Cloud Run and verify /v1/status shows sparsifier fields
• Verify existing brain operations unaffected

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