# Trust Decay / Freshness F(e) with Configurable Tau Values

## Module
VexLens

## Status
IMPLEMENTED

## Description
Freshness decay with configurable tau values per source class, implementing the F(e) = exp(-delta_days/tau) formula described in the advisory.

## Implementation Details
- **Trust decay service**: `src/VexLens/StellaOps.VexLens/Trust/SourceTrust/TrustDecayService.cs`, `ITrustDecayService.cs` -- manages trust decay calculations
- **Decay calculator**: `src/VexLens/StellaOps.VexLens/Trust/SourceTrust/TrustDecayCalculator.cs` -- implements F(e) = exp(-delta_days/tau) formula with configurable tau per source class
- **Trust weight engine**: `src/VexLens/StellaOps.VexLens/Trust/TrustWeightEngine.cs`, `ITrustWeightEngine.cs` -- integrates decay into trust weight computation
- **Core trust engine**: `src/VexLens/StellaOps.VexLens/StellaOps.VexLens.Core/Trust/TrustWeightEngine.cs`, `ITrustWeightEngine.cs`
- **VEX source trust score**: `src/VexLens/StellaOps.VexLens/Trust/SourceTrust/VexSourceTrustScore.cs` -- trust score model including freshness component
- **Trust score cache**: `src/VexLens/StellaOps.VexLens/Trust/SourceTrust/InMemorySourceTrustScoreCache.cs` -- caches computed trust scores
- **Options**: `src/VexLens/StellaOps.VexLens/Options/VexLensOptions.cs` -- configurable tau values per source class
- **Source**: Feature matrix scan

## E2E Test Plan
- [ ] Verify F(e) = exp(-delta_days/tau) produces correct decay for different delta values
- [ ] Test configurable tau per source class (vendor vs. community vs. unknown)
- [ ] Verify decay correctly reduces trust score for stale VEX statements
- [ ] Test fresh VEX statements maintain full trust weight
- [ ] Verify trust score cache invalidation on new statements