feat: implement core architecture modules

Phase 1: Wiki-Link Graph Indexing
- WikiLinkParser: extract [[links]] from markdown
- WikiLinkGraph: BFS traversal, reachable docs, backlinks
- Support relative path resolution (../../../)

Phase 2: ScoringPipeline trait (SOLID design)
- DocumentScorer trait: single interface for all scorers
- GlobalTfIdfScorer, ProjectTfIdfScorer, SemanticScorer
- MetadataBoostingScorer (decorator pattern)
- ScoringPipeline: orchestrate multiple scorers with RRF fusion
- Benefits: add new scorers without modifying existing code

Phase 7: RBAC + PolicyProvider trait
- PolicyProvider trait: pluggable backends (Vault, Postgres, Redis)
- VaultPolicyProvider: load YAML from vault/projects/* and vault/shared/skills/*
- MockPolicyProvider: for testing (no I/O)
- AccessChecker trait: single-purpose RBAC checks
- AccessLevelChecker, RoleChecker, PermissionChecker
- AccessDecisionEngine: orchestrate checkers with short-circuit eval
- AuditLogger trait: pluggable audit backends

Test Fixtures (DRY principle)
- OidcClaimsBuilder: fluent API for test data
- AccessPolicyBuilder: fluent API for policies
- MockPolicyProvider, MockAuditLogger: testing mocks

All modules compile and unit tests pass.

crates/mem-core/src/lib.rs

@@ -8,6 +8,7 @@ pub mod gate_parser;
 pub mod gated_loop;
 pub mod query_executor;
 pub mod optimizer;
+pub mod scoring;
 
 pub use gate_parser::{GateResponse, ParseError, parse_gate_response};
 
@@ -22,3 +23,4 @@ pub use query::{Query, QuerySet, SynthesisQuery};
 pub use prompt::{PromptBuilder, PromptMessages, CacheMetrics};
 pub use symptom_projection::{project_symptom, SymptomVector};
 pub use optimizer::{ContextOptimizer, ContextOptimizerConfig, ContentType, OptimizedChunk, CacheAligner, AlignedContent, CcrStore};
+pub use scoring::{DocumentScorer, ScoringPipeline, GlobalTfIdfScorer, ProjectTfIdfScorer, SemanticScorer, MetadataBoostingScorer};