test: split 50-smoke.sx into per-section examples + add errors smoke

Break the monolithic examples/50-smoke.sx into 30 focused per-section examples,
filed into their category blocks (basic/types/comptime/memory/protocols/ffi),
each carrying only the top-level decls its section references (the protocols
section keeps the full preamble — its deps flow through UFCS method calls that
name-based extraction can't see). Outputs verified identical to the original
section blocks.

Add examples/1036-errors-failable-smoke.sx — an end-to-end error-handling example
(the E5.4 work): named + inferred error sets consumed via destructure, try (in
helpers), catch (bare-expr / match-body / diverging / no-binding), or
value-terminator, onfail+defer interleave, and error.X value + {} tag
interpolation.

Remove examples/50-smoke.sx. Suite: 324 passed, 0 failed.

examples/0035-basic-array-loop-mutation.sx

@@ -0,0 +1,24 @@
+#import "modules/std.sx";
+#import "modules/math/math.sx";
+#import "modules/compiler.sx";
+#import "modules/test.sx";
+pkg :: #import "modules/testpkg";
+
+main :: () {
+
+    // ========================================================
+    // 25. ARRAY LOOP MUTATION
+    // ========================================================
+    print("=== 25. Array Loop Mutation ===\n");
+    {
+        arr : [4]s32 = .[0, 0, 0, 0];
+        i := 0;
+        while i < 4 {
+            arr[i] = xx (i + 1);
+            i += 1;
+        }
+        print("loop-fill: {} {} {} {}\n", arr[0], arr[1], arr[2], arr[3]);
+        arr[2] += 10;
+        print("compound: {}\n", arr[2]);
+    }
+}