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/diagnostics/expected/1107-diagnostics-ref-capture-value-arg-diagnostic.stderr

@@ -0,0 +1,10 @@
+error: by-reference loop capture 'm' has type '*Move', but 'Move' is expected here
+  --> examples/diagnostics/1107-diagnostics-ref-capture-value-arg-diagnostic.sx:15:14
+   |
+15 |         take(m);
+   |              ^
+
+help: dereference it to pass the value: `m.*`
+   |
+15 | m.*
+   |              ^