fixes #25611; t.state != Sealed (#25622)

fixes #25611

This pull request updates the `propagateToOwner` procedure in
`compiler/ast.nim` to handle sealed types more robustly during
incremental compilation (IC) reloads. The main change is the addition of
an assertion to ensure that sealed types already have the necessary
propagated flags, preventing incorrect state during IC reloads.

Handling of sealed types and propagated flags:

* Added a check for `Sealed` state on `o2` (the owner type), and
included an assertion to verify that sealed types already have the
required propagated flags (`tfHasAsgn`/`tfHasOwned`) during IC reloads,
instead of redundantly setting them.

Author: ringabout

compiler/ast.nim

@@ -1196,7 +1196,12 @@ proc propagateToOwner*(owner, elem: PType; propagateHasAsgn = true) =
     let o2 = owner.skipTypes({tyGenericInst, tyAlias, tySink})
     if o2.kind in {tyTuple, tyObject, tyArray,
                    tySequence, tyString, tySet, tyDistinct}:
-      o2.incl mask
+      if o2.state == Sealed:
+        # During the original compilation, propagateToOwner set tfHasAsgn/tfHasOwned on the type before it was sealed
+        # On IC reload, the sealed type already has those flags
+        assert mask <= o2.flags, "IC bug: sealed type missing propagated flags"
+      else:
+        o2.incl mask
       owner.incl mask
 
   if owner.kind notin {tyProc, tyGenericInst, tyGenericBody,