TBoldObjectReference accepts locators from another TBoldSystem

[bero]

Single-role references accept a locator that belongs to another TBoldSystem, while the list side (TBoldObjectList.CheckAdd/CheckReplace) rejects it. Found during a full-history code review (finding M2). Not a regression — the guard has been missing on the single-reference side since the original BoldSoft code.

Symptom

Assigning a single-role association across two live TBoldSystem instances (e.g. A1.next := A2 where A1 and A2 live in different systems) is silently accepted. The stored locator's ID belongs to the other system's database, so persisting the link writes a foreign ID — a dangling-link corruption once the two databases diverge.

Root cause

1. TBoldObjectList.CheckAdd/CheckReplace in Source/ObjectSpace/BORepresentation/BoldSystem.pas guard multilinks with OwnedByObject and (BoldSystem <> NewLocator.BoldSystem).
2. The single-reference counterpart TBoldObjectReference.CanSetLocator only calls VerifyClass — it never compares systems, so cross-system locators pass validation and are stored.

Fix

• TBoldObjectReference.CanSetLocator: after VerifyClass, reject when Assigned(NewLocator) and OwnedByObject and (BoldSystem <> NewLocator.BoldSystem), setting a failure reason so SetLocator's existing BoldRaiseLastFailure turns it into a raised exception — same mechanism as the list side.
• BoldCoreConsts: new sCannotLinkAcrossSystems message constant.
• Test: TestSetLocatorRejectsCrossSystemObject in UnitTest/Code/ObjectSpace/Test.BoldSystem.pas — two live systems sharing one SystemTypeInfoHandle; verifies the assignment raises and nothing is stored (red before the fix, green after).

Files changed

• Source/ObjectSpace/BORepresentation/BoldSystem.pas
• Source/ObjectSpace/Core/BoldCoreConsts.pas
• UnitTest/Code/ObjectSpace/Test.BoldSystem.pas

Testing

• New regression test fails without the guard ("Method did not throw any exceptions.") and passes with it
• Full suite: 2120 found / 2118 pass / 0 failures — no legitimate usage broken