ios: framework embedding in .app + -F search paths

- `-F <dir>` CLI flag adds Apple framework search paths (parallel to `-L`).
- `TargetConfig.framework_paths` flows into the iOS link line (`-F<dir>`).
- iOS link adds `-Wl,-rpath,@executable_path/Frameworks` so embedded
  frameworks resolve at runtime.
- `createBundle` now takes the framework list; for each one it locates
  `<name>.framework` in the `-F` paths and `cp -R`s it into
  `<bundle>.app/Frameworks/`.
- `c_import.compileCToObjects` forwards `-target`/`-isysroot` to clang so
  `#c_import` works under cross-compile (was using host clang implicitly).
- iOS SDK is auto-discovered once at startup and shared by both the C
  compile and the link paths.
- `SX_DEBUG_LINK=1` prints the resolved link argv.
- `library/modules/sdl3.sx`: drop `#library "SDL3"` — linking is now
  per-target (build.sx handles `-lSDL3` on macOS, `-framework SDL3` on iOS).

src/c_import.zig

@@ -175,6 +175,7 @@ pub fn processCImport(
 pub fn compileCToObjects(
     allocator: std.mem.Allocator,
     infos: []const CImportInfo,
+    target_config: @import("target.zig").TargetConfig,
 ) ![]c.LLVMMemoryBufferRef {
     var obj_bufs = std.ArrayList(c.LLVMMemoryBufferRef).empty;
 
@@ -183,6 +184,15 @@ pub fn compileCToObjects(
 
         // Build clang args: -I dirs, -D defines, raw flags
         var args_list = std.ArrayList([*c]const u8).empty;
+        // Cross-compile target: forward -target / -isysroot when set.
+        if (target_config.triple) |t| {
+            try args_list.append(allocator, "-target");
+            try args_list.append(allocator, t);
+        }
+        if (target_config.sysroot) |sr| {
+            try args_list.append(allocator, "-isysroot");
+            try args_list.append(allocator, (try allocator.dupeZ(u8, sr)).ptr);
+        }
         for (info.includes) |inc| {
             const dir = dirName(inc);
             try args_list.append(allocator, (try allocPrintZ(allocator, "-I{s}", .{dir})).ptr);