chore: fix all build warnings and add stabilization roadmap

- Fix 3× ResultShadowed in cypher.nim, recovery.nim, shell.nim
- Remove 6× UnusedImport in test files
- Build is now clean: 0 warnings, 294 tests pass
- Add 4-week stabilization plan to PLAN.md (Sessions 9.1–9.4)

src/barabadb/graph/cypher.nim

@@ -212,7 +212,6 @@ proc executeCypher*(g: Graph, query: CypherQuery): CypherResult =
 
 proc toCypher*(query: string): string =
   ## Convert basic BaraQL to Cypher-like syntax for graph queries
-  var result = ""
   let upper = query.toUpper()
   if upper.startsWith("SELECT") and upper.contains("MATCH"):
     # Already Cypher-like