fix: union struct-literal init (issue 0158)

A plain union initialized with a struct literal (b : Overlay = .{ f = 3.14 })
silently miscompiled — it fell through the generic struct-literal path
(getStructFields returns empty for a union), building a malformed structInit
whose overlapping zero-fill clobbered the named member, so it read back 0.0
(and a type-pun read segfaulted).

lowerStructLiteral now detects a plain-union target and dispatches to a new
lowerUnionLiteral, which writes each named member into a union-sized slot via
the same lvalue resolver the u.member = v assignment path uses, then loads the
union value back. Validity: the named members must share one arm — a single
direct member, or several promoted members of the same anonymous-struct variant.
Overlapping members, members from different arms, and positional union literals
are rejected with a diagnostic (no silent last-wins); an empty .{} yields an
undefined union (matching the --- form).

specs.md updated. Regressions: examples/types/0194 (valid forms) +
examples/diagnostics/1191 (overlap rejection).

src/ir/lower.zig

@@ -1661,6 +1661,7 @@ pub const Lowering = struct {
     pub const lowerAssignment = lower_stmt.lowerAssignment;
     pub const fieldLvalueResolve = lower_stmt.fieldLvalueResolve;
     pub const fieldLvaluePtr = lower_stmt.fieldLvaluePtr;
+    pub const lowerUnionLiteral = lower_stmt.lowerUnionLiteral;
     pub const diagTaggedUnionVariantWrite = lower_stmt.diagTaggedUnionVariantWrite;
     pub const lowerExprAsPtr = lower_stmt.lowerExprAsPtr;
     pub const storeOrCompound = lower_stmt.storeOrCompound;