Implement VEX document verification system with issuer management and signature verification

- Added IIssuerDirectory interface for managing VEX document issuers, including methods for registration, revocation, and trust validation.
- Created InMemoryIssuerDirectory class as an in-memory implementation of IIssuerDirectory for testing and single-instance deployments.
- Introduced ISignatureVerifier interface for verifying signatures on VEX documents, with support for multiple signature formats.
- Developed SignatureVerifier class as the default implementation of ISignatureVerifier, allowing extensibility for different signature formats.
- Implemented handlers for DSSE and JWS signature formats, including methods for verification and signature extraction.
- Defined various records and enums for issuer and signature metadata, enhancing the structure and clarity of the verification process.

etc/vexlens.yaml.sample

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+# VexLens Configuration Sample
+# Copy to vexlens.yaml and customize for your environment
+
+VexLens:
+  # Storage configuration for consensus projections
+  Storage:
+    # Driver: "memory" for testing, "mongo" for production
+    Driver: mongo
+    ConnectionString: "mongodb://localhost:27017"
+    Database: stellaops
+    ProjectionsCollection: vex_consensus
+    HistoryCollection: vex_consensus_history
+    MaxHistoryEntries: 100
+    CommandTimeoutSeconds: 30
+
+  # Trust engine configuration
+  Trust:
+    # Base weights by issuer trust tier (0.0-1.0)
+    AuthoritativeWeight: 1.0   # Authoritative sources (e.g., product vendors)
+    TrustedWeight: 0.8         # Trusted third parties
+    KnownWeight: 0.5           # Known but not verified
+    UnknownWeight: 0.3         # Unknown sources
+    UntrustedWeight: 0.1       # Untrusted/unverified sources
+
+    # Weight multiplier for cryptographically signed statements
+    SignedMultiplier: 1.2
+
+    # Freshness decay: statements older than this start losing weight
+    FreshnessDecayDays: 30
+    MinFreshnessFactor: 0.5    # Minimum freshness factor (0.0-1.0)
+
+    # Status-specific boosts
+    JustifiedNotAffectedBoost: 1.1  # Boost for not_affected with justification
+    FixedStatusBoost: 1.05          # Boost for fixed status
+
+  # Consensus computation configuration
+  Consensus:
+    # Mode: HighestWeight, WeightedVote, Lattice, AuthoritativeFirst
+    DefaultMode: WeightedVote
+
+    # Minimum weight for a statement to contribute
+    MinimumWeightThreshold: 0.1
+
+    # Weight difference to trigger conflict detection
+    ConflictThreshold: 0.3
+
+    # Require justification for not_affected status
+    RequireJustificationForNotAffected: false
+
+    # Maximum statements per computation (performance limit)
+    MaxStatementsPerComputation: 100
+
+    # Enable conflict detection and reporting
+    EnableConflictDetection: true
+
+    # Emit events on consensus changes
+    EmitEvents: true
+
+  # Normalization configuration
+  Normalization:
+    # Enabled VEX format normalizers
+    EnabledFormats:
+      - OpenVEX
+      - CSAF
+      - CycloneDX
+
+    # Fail on unknown fields (strict mode)
+    StrictMode: false
+
+    # Size limits
+    MaxDocumentSizeBytes: 10485760   # 10 MB
+    MaxStatementsPerDocument: 10000
+
+  # Air-gap mode configuration
+  AirGap:
+    # Enable sealed mode (block external network access)
+    SealedMode: false
+
+    # Path to offline bundle directory
+    BundlePath: /var/lib/stellaops/vex-bundles
+
+    # Verify bundle signatures on import
+    VerifyBundleSignatures: true
+
+    # Allowed bundle sources (issuer IDs)
+    AllowedBundleSources: []
+
+    # Export format: jsonl, json
+    ExportFormat: jsonl
+
+  # Telemetry configuration
+  Telemetry:
+    MetricsEnabled: true
+    TracingEnabled: true
+    MeterName: StellaOps.VexLens
+    ActivitySourceName: StellaOps.VexLens
+
+# Logging configuration (optional override)
+Logging:
+  LogLevel:
+    Default: Information
+    StellaOps.VexLens: Debug
+
+# OpenTelemetry configuration (when telemetry enabled)
+# OpenTelemetry:
+#   Endpoint: http://localhost:4317
+#   Protocol: grpc