issues: relocate legacy examples/issue-* repros into issues/

Clear the examples/issue-* namespace (new layout keeps open-issue repros under
issues/, co-located with their .md). Two legacy files:

- issue-0030 was a feature-request placeholder (trivial main, no real test).
  `extern G : T;` cross-file sx globals are still unimplemented (parse error),
  so it's an open feature request: issues/0030-extern-global-declarations.{md,sx}.
- issue-0019 was a broken/superseded multi-file fixture (relative imports, not
  runnable from root; the non-transitive-#import scenario is covered by the
  passing 0706-modules-import-non-transitive). Moved to
  issues/0019-import-non-transitive-c-scope/ with a status note; safe to delete.

Suite unchanged: 324 passed.

issues/0030-extern-global-declarations.sx

@@ -0,0 +1,14 @@
+// Repro for issue 0030 (OPEN feature request): cross-file sx `extern` globals.
+// Want: `extern G : T;` declares a reference to a global defined in another sx
+// file (resolved at link time), mirroring `#foreign` functions. Today this is a
+// parse error — the form doesn't exist. Distinct from `name : T #foreign;`
+// (an external C data symbol; see examples/1205-ffi-foreign-global.sx).
+//
+// Expected (once implemented): parses; `g_x` resolves to a global defined
+// elsewhere. Actual: error "expected '::', ':=', or ':' after identifier".
+
+#import "modules/std.sx";
+
+extern g_x : *void;
+
+main :: () -> s32 { 0; }