fix(0139): reject by-value self-referential types loudly (was a segfault)

A nominal aggregate that contains itself (or a mutual peer) BY VALUE has no
finite layout and infinite-recursed typeSizeBytes into a stack overflow —
for SOURCE enums/structs as well as comptime-constructed types.

New `checkInfiniteSize` pass (lower/decl.zig, Pass 1g — after type
registration, before body lowering): walks the by-VALUE containment graph
(pointer/slice/optional payloads break the cycle, so `*Self` stays valid);
on a back-edge it emits a loud diagnostic — "type 'X' is infinitely sized
(it contains itself by value); use a pointer ('*X') to break the cycle" —
and poisons the offending field to `.unresolved` so sizing can't recurse
before the build halts on the error. Covers source + declare/define types,
direct + mutual recursion.

examples/1178 locks the diagnostic; issue 0139 marked RESOLVED. This also
completes METATYPE PLAN F5's by-value-self-reference rejection. Full suite
green (675).

src/ir/lower.zig

@@ -1642,6 +1642,10 @@ pub const Lowering = struct {
     pub const ensureTerminator = lower_control_flow.ensureTerminator;
 
     // --- moved to lower/decl.zig (lower_decl) ---
+    pub const checkInfiniteSize = lower_decl.checkInfiniteSize;
+    pub const dfsByValueCycle = lower_decl.dfsByValueCycle;
+    pub const poisonAggregateField = lower_decl.poisonAggregateField;
+    pub const diagInfiniteSize = lower_decl.diagInfiniteSize;
     pub const SelectedFunc = lower_decl.SelectedFunc;
     pub const BareCallee = lower_decl.BareCallee;
     pub const VisibleStructAuthor = lower_decl.VisibleStructAuthor;