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).

examples/1178-diagnostics-infinite-size-self-reference.sx

@@ -0,0 +1,16 @@
+// Diagnostic: a type that contains ITSELF by value has no finite size and must
+// be rejected loudly (not infinite-loop the size computation into a crash). A
+// pointer payload (`*Tree`) would break the cycle and is the fix the message
+// suggests. Covers both source decls and comptime-constructed types — this is
+// the source form (regression for issue 0139).
+#import "modules/std.sx";
+
+Tree :: enum {
+    node: Tree;     // by-VALUE self-reference → infinitely sized
+    leaf;
+}
+
+main :: () -> i32 {
+    t : Tree = .leaf;
+    return 0;
+}

examples/expected/1178-diagnostics-infinite-size-self-reference.exit

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

examples/expected/1178-diagnostics-infinite-size-self-reference.stderr

@@ -0,0 +1 @@
+error: type 'Tree' is infinitely sized (it contains itself by value); use a pointer ('*Tree') to break the cycle

examples/expected/1178-diagnostics-infinite-size-self-reference.stdout

@@ -0,0 +1 @@
+