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/expected/1026-errors-failable-main.stderr

@@ -0,0 +1,8 @@
+error: unhandled error reached main: error.Empty
+error return trace (most recent call last):
+  inner at 1026-errors-failable-main.sx:18:17
+        if n == 0 { raise error.Empty; }       // pushes a frame
+                    ^
+  main at 1026-errors-failable-main.sx:26:10
+        w := try inner(0);                       // raises Empty → propagates to main
+             ^