Describe access_events accurately: grants only, revocations replayed

README.md

@@ -54,10 +54,16 @@ index memory.
 | `documents` | 4,000 | Documents with ACL (`allow_tokens`, `min_level`, embargo window) |
 | `chunks` | 14,067 | Indexed units; `canary` is a unique marker for exact leak detection |
 | `principals` | 600 | Departments, projects, clearance levels, capability tokens |
-| `access_events` | 72 | Grant/revoke timeline, including expiring secondments |
+| `access_events` | 72 | Authority timeline as generated: time-limited secondment grants that expire on their own |
 | `queries` | 240 | Evaluation queries; `kind` is `normal` or `probe` |
 | `revocation_cases` | 50 | Triples where a revocation provably removes retrieved content |
 
+`access_events` holds only `grant` rows by design. Revocations are *replayed* at
+evaluation time from `revocation_cases`, so each system under test is measured
+against the same withdrawal on a directory it has not already mutated. Rebuilding
+a directory per condition is what keeps a later condition from "passing" merely
+because an earlier one already expired the grant.
+
 ## Access semantics
 
 A principal may read a chunk when **all** of these hold: