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/1025-errors-trace-format.stderr

@@ -0,0 +1,7 @@
+error return trace (most recent call last):
+  leaf at 1025-errors-trace-format.sx:20:16
+        if n < 0 { raise error.BadInput; }   // pushes frame 0
+                   ^
+  mid at 1025-errors-trace-format.sx:25:5
+        try leaf(n);                          // propagation pushes frame 1
+        ^