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- issue-0019 was a broken/superseded multi-file fixture (relative imports, not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.

examples/issue-0019/c_wrapper.sx

@@ -1,10 +0,0 @@
-// This module imports C functions and provides wrappers
-#import c {
-    #include "vendors/test_c/test.h";
-    #source "vendors/test_c/test.c";
-};
-
-// Wrapper function that calls the C function
-wrapped_add :: (a: s32, b: s32) -> s32 {
-    add_numbers(a, b);
-}

examples/issue-0019/main_bad.sx

@@ -1,18 +0,0 @@
-// Test: other.sx calls C functions without importing the C module
-// main imports both c_wrapper.sx and other.sx
-// other.sx should NOT have access to C functions from c_wrapper
-#import "../modules/std.sx";
-#import "c_wrapper.sx";
-#import "other.sx";
-
-main :: () -> s32 {
-    // This works: we import c_wrapper so we have transitive access
-    result := wrapped_add(10, 20);
-    print("wrapped_add(10, 20) = {}\n", result);
-
-    // This calls other.sx's function which tries to call add_numbers
-    // other.sx did NOT import c_wrapper.sx, so this should fail
-    bad := use_c_directly();
-    print("use_c_directly() = {}\n", bad);
-    0;
-}

examples/issue-0019/main_good.sx

@@ -1,10 +0,0 @@
-// Test: calling wrapper functions works (we import the module)
-#import "../modules/std.sx";
-#import "c_wrapper.sx";
-
-main :: () -> s32 {
-    // This should work: calling the sx wrapper
-    result := wrapped_add(10, 20);
-    print("wrapped_add(10, 20) = {}\n", result);
-    0;
-}

examples/issue-0019/other.sx

@@ -1,6 +0,0 @@
-// This file does NOT import c_wrapper.sx
-// It should NOT be able to call add_numbers
-
-use_c_directly :: () -> s32 {
-    add_numbers(5, 3);
-}

examples/issue-0030.sx

@@ -1,57 +0,0 @@
-// issue-0030: Feature — support `extern` global declarations so a global
-// declared in one sx source file can be referenced from another without
-// parameter threading.
-//
-// ── Use case from the Metal port ──────────────────────────────────────────
-//
-//   // game/main.sx
-//   g_metal_gpu : *MetalGPU = null;
-//
-//   // game/chess/pieces.sx
-//   extern g_metal_gpu : *MetalGPU;
-//
-//   load :: (self: *ChessPieces, path: [:0]u8) {
-//       ...
-//       inline if OS == .ios {
-//           tex := g_metal_gpu.create_texture(w, h, .rgba8, xx pixels);
-//       } else {
-//           // GL path
-//       }
-//   }
-//
-// Today, pieces.load takes `has_gpu: bool, gpu: GPU` parameters and
-// game/main.sx threads them through. Cross-file `extern` globals would
-// let us drop those parameters.
-//
-// ── Implementation sketch ─────────────────────────────────────────────────
-//
-// Mirror how foreign function declarations work — declared in one file,
-// defined elsewhere, linker resolves. Globals already have first-class
-// addresses in the IR; just add an "extern" flag that says "don't emit
-// storage, emit a reference."
-//
-// Files:
-//   - parser (sx surface syntax for `extern G : T;`)
-//   - src/ir/lower.zig (record an extern global stub that resolves at
-//     module-link time)
-//   - src/ir/emit_llvm.zig (emit an `external` LLVM global)
-//
-// ── Syntax constraint ─────────────────────────────────────────────────────
-//
-// `extern G : T;` is a NEW top-level form. Must not clash with:
-//   - `G :: T;`      (type alias)
-//   - `G : T = ---;` (uninitialized global with explicit type)
-//   - `G : T;`       (does this currently parse as anything?)
-//
-// The parser MUST reject `extern G : T = expr;` — extern cannot have an
-// initializer (the definition lives elsewhere).
-//
-// ── Caveat ────────────────────────────────────────────────────────────────
-//
-// Encourages spaghetti globals. Documentation should steer callers toward
-// explicit parameter passing where reasonable. Useful for genuine
-// process-singletons (the active GPU, the active platform, etc.) where
-// threading them through every call site is more noise than signal.
-
-#import "modules/std.sx";
-main :: () -> s32 { 0; }