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)

tests/join_tests.nim

@@ -1,13 +1,10 @@
 import std/unittest
 import std/os
 import std/strutils
-import std/times
 import std/monotimes
 import std/tables
-import barabadb/core/types
 import barabadb/query/executor as qexec
 import barabadb/query/parser
-import barabadb/query/ast
 import barabadb/storage/lsm
 
 proc execSql(ctx: qexec.ExecutionContext, sql: string): qexec.ExecResult =