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/0307-closures-closure-contextual-params.sx

@@ -0,0 +1,28 @@
+// Step 2.5 — contextual typing for closure literals with N (heterogeneous)
+// params. An untyped lambda `(a, b, c) => ...` takes each param's type
+// positionally from the expected `Closure(T0, T1, T2) -> R` signature, in both
+// assignment and argument position. (Previously only the first param — or
+// all-same-typed params — resolved; trailing params silently defaulted to s64.)
+
+#import "modules/std.sx";
+
+// argument-position: lambda typed from the parameter's closure type.
+apply2 :: (f: Closure(s64, string) -> s64, x: s64, s: string) -> s64 {
+    return f(x, s);
+}
+apply3 :: (f: Closure(s64, s64, string) -> s64, a: s64, b: s64, c: string) -> s64 {
+    return f(a, b, c);
+}
+
+main :: () -> s32 {
+    // assignment-position, mixed (s64, string) params — `b` is string.
+    cb : Closure(s64, string) -> s64 = (a, b) => a + b.len;
+    print("cb={}\n", cb(10, "hello"));     // 10 + 5 = 15
+
+    // argument-position, 2 params.
+    print("r={}\n", apply2((a, b) => a + b.len, 10, "hello"));   // 15
+
+    // argument-position, 3 params (s64, s64, string).
+    print("q={}\n", apply3((a, b, c) => a + b + c.len, 1, 2, "xyz"));  // 1+2+3 = 6
+    0;
+}