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).

examples/diagnostics/expected/1191-diagnostics-union-literal-overlap.exit

@@ -0,0 +1 @@
+1

examples/diagnostics/expected/1191-diagnostics-union-literal-overlap.stderr

@@ -0,0 +1,5 @@
+error: a union literal may set only one member, or several members of the same anonymous-struct arm — 'Overlay's members overlay the same storage
+  --> examples/diagnostics/1191-diagnostics-union-literal-overlap.sx:11:19
+   |
+11 |     o : Overlay = .{ f = 3.14, i = 7 };   // ERROR: f and i overlay the same bytes
+   |                   ^^^^^^^^^^^^^^^^^^^^

examples/diagnostics/expected/1191-diagnostics-union-literal-overlap.stdout

@@ -0,0 +1 @@
+