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/ffi-objc/1304-ffi-objc-block-multi-arg.sx

@@ -0,0 +1,61 @@
+// `xx closure : Block` for an arbitrary closure signature.
+//
+// The stdlib (modules/ffi/objc_block.sx) declares hand-rolled
+// `Into(Block) for Closure() -> void` and `Closure(bool) -> void`
+// impls — the two most common Apple block shapes. Other signatures
+// need a per-shape `__block_invoke_<sig>` trampoline + `Into(Block)`
+// impl declared somewhere reachable (stdlib if shared, in-file if
+// app-specific).
+//
+// This test exercises the user-declared variant: signature
+// `Closure(i32, *void) -> void` (a two-arg block — not in stdlib).
+// If the impl is missing, the compiler emits a focused diagnostic
+// pointing at modules/ffi/objc_block.sx as the template.
+
+#import "modules/std.sx";
+#import "modules/ffi/objc_block.sx";
+
+// Trampoline matching `void (^)(int, void*)` — the C ABI Apple's
+// runtime calls. Forwards through to the sx closure with the
+// standard `(__sx_ctx, env, ...args)` shape.
+__block_invoke_void_i32_p :: (block_self: *Block, arg0: i32, arg1: *void) abi(.c) {
+    typed_fn : (*void, i32, *void) -> void = xx block_self.sx_fn;
+    typed_fn(block_self.sx_env, arg0, arg1);
+}
+
+impl Into(Block) for Closure(i32, *void) -> void {
+    convert :: (self: Closure(i32, *void) -> void) -> Block {
+        .{
+            isa = @_NSConcreteStackBlock,
+            flags = 0,
+            reserved = 0,
+            invoke = xx @__block_invoke_void_i32_p,
+            descriptor = xx @__sx_block_descriptor,
+            sx_env = self.env,
+            sx_fn = self.fn_ptr,
+        }
+    }
+}
+
+// Side-effect capture so we can observe both args reached the
+// closure body.
+g_sum: i32 = 0;
+g_tag: *void = null;
+
+main :: () -> i32 {
+    cl := (n: i32, tag: *void) => {
+        g_sum = n + 1;
+        g_tag = tag;
+    };
+    b : Block = xx cl;
+
+    invoke_fn : (*Block, i32, *void) -> void abi(.c) = xx b.invoke;
+    sentinel: i32 = 42;
+    invoke_fn(@b, 41, xx @sentinel);
+
+    if g_sum != 42 { print("FAIL: g_sum expected 42, got {}\n", g_sum); return 1; }
+    if g_tag == null { print("FAIL: g_tag null\n"); return 1; }
+
+    print("block multi-arg ok: sum={}\n", g_sum);
+    0
+}