test: migrate examples to XXXX-category-name layout + split expected streams

Rename all example tests/companions to the XXXX-category-test-name scheme
(per-category 100-blocks: basic 0010, types 0100, ... errors 1000,
diagnostics 1100, ffi 1200, ffi-objc 1300, ffi-jni 1400, vectors 1500,
platform 1600). Companions and dir/C fixtures move in lockstep with their
parent test; #import/#source/#include paths rewritten to match.

Expected output now lives in examples/expected/ (a sibling dir of the
tests) split into three streams per the new convention:
  <name>.exit / <name>.stdout / <name>.stderr  (+ optional <name>.ir)

run_examples.sh rewritten: scans examples/ and issues/ for an
expected/<name>.exit marker, captures stdout and stderr separately (no
more 2>&1), compares each stream + exit + optional IR snapshot.

Behavior validated unchanged: every renamed test reproduces its prior
merged output + exit (diffs limited to file paths/basenames embedded in
diagnostics + traces, which correctly reflect the new names). Suite:
292 passed, 0 failed. 50-smoke.sx split + issue relocation + docs follow
in subsequent commits.

examples/1421-ffi-jni-env-02-lexical-direct.sx

@@ -0,0 +1,30 @@
+// Phase 2 step 2.16b (PLAN-FFI.md): xfail then green for the
+// lexical-direct env resolution inside a `#jni_env` scope.
+//
+// `#jni_call(T)(target, "name", "sig", args...)` — three args before
+// the first string literal — has the env omitted. The lowering walks
+// the in-progress AST visit stack to find the enclosing `#jni_env(env)`
+// block and uses that env value directly as the IR-level env arg.
+// No thread-local read, no per-call lookup; env stays register-
+// resident across loop bodies (the hot-path optimisation).
+//
+// Today's lower expects 4+ args and errors when called with 3.
+
+#import "modules/std.sx";
+
+g_should_call : bool = false;
+
+unused_jni :: (env: *void, target: *void) {
+    #jni_env(env) {
+        // Omitted env — env comes from the enclosing #jni_env scope.
+        #jni_call(void)(target, "noop", "()V");
+    }
+}
+
+main :: () -> s32 {
+    if g_should_call {
+        unused_jni(null, null);
+    }
+    print("ok\n");
+    0;
+}