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/errors/1026-errors-failable-main.sx

@@ -0,0 +1,29 @@
+// Failable `-> !` main entry-point wrapper (ERR step E4.2). A pure-failable
+// main that lets an error reach the function boundary exits 1 and prints the
+// unhandled-error header (with the tag name, via the always-linked tag-name
+// table) plus the return trace to stderr — instead of the old behavior of
+// returning the raw tag id as the exit code with no diagnostic. A successful
+// run (no escaping error) exits 0.
+//
+// Note: the header + trace go to stderr. The test runner merges stderr+stdout,
+// so the snapshot shows them interleaved with the `print` (stdout) lines.
+// Frame locations are placeholders until DWARF (ERR E3.0); count + ordering +
+// the tag name are already meaningful. Expected exit code: 1.
+
+#import "modules/std.sx";
+
+ParseErr :: error { Empty, BadDigit };
+
+inner :: (n: i32) -> (i32, !ParseErr) {
+    if n == 0 { raise error.Empty; }       // pushes a frame
+    if n < 0 { raise error.BadDigit; }
+    return n * 2;
+}
+
+main :: () -> !ParseErr {
+    v := try inner(5);                       // succeeds → v = 10
+    print("v = {}\n", v);
+    w := try inner(0);                       // raises Empty → propagates to main
+    print("w = {}\n", w);                    // never reached
+    return;
+}