Two related set/enum fixes from shaking the sets cluster.
1. Set subset (<=) and superset (>=) operators, e.g. `if s <= t`:
- Semantic: <= and >= on two sets yield Boolean (alongside =, <>).
- Codegen both backends: non-jumbo sets test (s and not t)=0 for <=
(operands swapped for >=); jumbo sets call a new RTL _SetSubset
(added to blaise_set.pas) which returns whether every bit of A is in B.
Completes the set-operator suite (+, -, *, in, =, <>, <=, >= now all work).
2. Reject a variable that shadows a visible enum member (e.g. `var c` when
an enum member `C` is visible — Pascal is case-insensitive). Such a
shadow silently retargeted the member in a set literal `[A, C, D]` to the
variable: QBE errored with "Set literal element 'c' is not a constant",
and — worse — the native backend produced a WRONG bitmask with no error
(the member's bit was dropped). This surfaced as a corrupt for-in over a
set when the loop variable collided with a member. Now rejected at the
declaration with a clear message, mirroring the existing type-name-shadow
rule (FPC/Delphi allow the shadow; Blaise does not).
Verified subset/superset truth tables and for-in over a set (no shadow) on
both backends, and that the shadow is now a compile error. Adds
TE2ESetOpsTests.TestRun_Set_SubsetSuperset and
TSemanticTests.TestVarDecl_ShadowsEnumMember_RaisesError. Rationale's
"Variable Names May Not Shadow Type Names" section extended to enum members.
07a1236896