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/1206-ffi-medium-struct.sx

@@ -0,0 +1,33 @@
+// 16-byte integer-only struct passed by value through `#foreign`.
+//
+// emit_llvm.zig's `abiCoerceParamType` routes 9..16-byte non-HFA
+// structs through `[2 x i64]` for register-pair passing on AAPCS64 /
+// SysV AMD64. The call-site side loads the sx struct as `{ i64, i64 }`,
+// so the verifier needs a memory-bitcast bridge in both directions
+// (`abi.struct2arr` on the way in, `abi.arr2struct` on the way back).
+//
+// Different ABI path from 8-byte register-pair structs (coerce to
+// i64), HFAs at the same total size (Vec4f stays a struct), and
+// >16-byte aggregates (passed via pointer + byval).
+//
+// This file was originally filed as issue-0036 (LLVM verifier
+// failure repro). Promoted to a focused feature example once the
+// emit_llvm.zig coerceArg branches landed.
+
+#import "modules/std.sx";
+
+#import c {
+    #source "1206-ffi-medium-struct.c";
+};
+
+Pair64 :: struct { a: s64; b: s64; }
+
+ffi_pair64_swap :: (p: Pair64) -> Pair64 #foreign;
+
+main :: () -> s32 {
+    p : Pair64 = .{ a = 1, b = 2 };
+    q := ffi_pair64_swap(p);
+    print("swap = ({}, {})\n", q.a, q.b);
+    print("ok = {}\n", q.a == 2 and q.b == 1);
+    0;
+}