platform: SDL3 backend (desktop + WASM) + two bug repros

- library/modules/platform/sdl3.sx: SdlPlatform impl wrapping SDL3 init,
  GL context, event pump, swap. run_frame_loop owns the loop: while loop
  on desktop, emscripten_set_main_loop on WASM. Registers an event-watch
  that re-invokes the frame closure during macOS modal resize-drag so
  content keeps rendering at the new size. safe_insets / keyboard /
  show_keyboard / hide_keyboard are no-ops (these targets have no soft
  keyboard).

Two compiler bug repros uncovered during the refactor:

- examples/issue-0020.sx: global `Foo = .{}` zero-initializes, ignoring
  struct field defaults. Local `Foo = .{}` correctly applies defaults.
  Workaround: set fields explicitly in an init method or heap-allocate
  the value.

- examples/issue-0021.sx: an enclosing function's return type bleeds
  into `xx`'s target type inside an `if-then-else` expression on the
  RHS of a struct-field assignment. The same expression in a `-> void`
  function produces the right value; in a `-> bool` function it
  silently produces 0. Bit the SX Chess game's dpi_scale calc inside
  `SdlPlatform.init` (returns bool), making all text labels render
  invisibly on retina. Workaround: hoist each `xx` cast into its own
  f32 local.

Regression gate: 50/50 examples pass, macOS chess game runs at ~2700fps
(close to the pre-refactor 2900 baseline), WASM build still emits a
working .html/.js/.wasm/.data quad.

examples/issue-0020.sx

@@ -0,0 +1,54 @@
+// issue-0020: Global `Foo = .{}` zero-initializes, ignoring field defaults
+//
+// Struct field defaults declared via `field: T = expr;` are honored when the
+// struct is constructed at function-local scope, but are silently dropped
+// when the struct is declared at module scope with `= .{}`.
+//
+// Repro:
+//
+//   Foo :: struct {
+//       running: bool = true;     // default
+//   }
+//
+//   g_foo : Foo = .{};            // global  → g_foo.running == false (BUG)
+//
+//   main :: () {
+//       l_foo : Foo = .{};        // local   → l_foo.running == true  (correct)
+//   }
+//
+// Surface bites:
+//
+//   - In the SX Chess game, the SDL3 platform backend stores a `running: bool
+//     = true` field on `SdlPlatform`. With `g_plat : SdlPlatform = .{};` the
+//     main loop's `while self.running { ... }` exits immediately because
+//     `running` was zero-initialized despite the field default.
+//   - Workaround: assign defaults explicitly in the type's `init` method, or
+//     spell every field out at the global construction site:
+//         g_plat : SdlPlatform = .{ running = true };
+//
+// Likely cause: the globals path emits an LLVM ConstantAggregateZero (or
+// memset-to-zero) for the initializer, skipping the per-field default-expr
+// lowering used for local declarations.
+//
+// This file is a runnable repro: locals print "running=true", globals print
+// "running=false".
+
+#import "modules/std.sx";
+
+Foo :: struct {
+    running: bool = true;
+    x: s32 = 42;
+}
+
+g_foo : Foo = .{};
+
+main :: () -> void {
+    out("global  running=");
+    out(if g_foo.running then "true" else "false");
+    out("\n");
+
+    l_foo : Foo = .{};
+    out("local   running=");
+    out(if l_foo.running then "true" else "false");
+    out("\n");
+}