fix+refactor: soft keywords as identifiers, full test wiring, ORC crash docs

- parser: clause keywords (header, format, status, user, csv, ...) now work
  as identifiers everywhere; IMPORT/EXPORT accept FORMAT csv/HEADER true
- nimble test + CI run all 13 test suites (650 checks green)
- ExecutionContext.registry is now {.cursor.} (breaks registry<->ctx cycle)
- ORC crash reproduced and bisected (tests/orc_repro.py); ARC stays the MM

src/barabadb/query/exec/types.nim

@@ -113,7 +113,12 @@ type
     outerRow*: Table[string, string]  # outer query row for correlated subqueries
     subqueryPlan*: IRPlan  # current subquery plan being evaluated
     currentDatabase*: string  # name of the currently selected database
-    registry*: DatabaseRegistry  # nil for single-DB mode
+    # The registry owns this context (registry -> DatabaseInfo -> ctx), so this
+    # back-reference is a non-owning cursor: it breaks the registry <-> ctx
+    # reference cycle. The registry always outlives its contexts (closeAll at
+    # shutdown). Note: breaking this cycle alone does NOT make ORC usable —
+    # the ORC crash under wire INSERT load persists (see tests/orc_repro.py).
+    registry* {.cursor.}: DatabaseRegistry  # nil for single-DB mode
 
   MigrationRecord* = object
     name*: string