test: group examples into per-category folders

Move examples/*.sx and their expected/ snapshots into per-category
subfolders (examples/<category>/...). Folder = leading filename token,
with ffi-objc/ffi-jni kept whole; filenames are unchanged. The corpus
runner and LSP sweep now discover each category's expected/ dir, while
issues/ stays flat. Example 1058's repo-root-relative companion import
is made file-relative. Path strings embedded in 164 snapshots were
regenerated (path-only changes). Test-layout docs in CLAUDE.md updated.

examples/platform/1664-platform-on-build-callback.sx

@@ -0,0 +1,21 @@
+#import "modules/std.sx";
+#import "modules/build.sx";
+
+// P5.4 — the `on_build` override. A user `#run on_build(build);` replaces the
+// stdlib `default_pipeline` as the build driver; the compiler invokes it after
+// codegen with the `BuildOptions` handle. This build GROWS a `List` on the build
+// VM (comptime List growth works only on the VM — issue 0141; the legacy interp
+// fails it) then DELEGATES to `default_pipeline` for the real emit + link, so a
+// working binary is still produced → "runtime main".
+
+build :: (opt: BuildOptions) -> bool abi(.compiler) {
+    names := List(string).{};
+    names.append("alpha");
+    names.append("beta");
+    names.append("gamma");
+    if names.len != 3 { return false; }
+    return default_pipeline(opt);
+}
+#run on_build(build);
+
+main :: () { print("runtime main\n"); }