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/basic/expected/0041-basic-block-value-reject.stderr

@@ -0,0 +1,5 @@
+error: function returns 'i32' but the last expression's value is discarded by this `;` — drop the `;` to return it (or use an explicit `return`)
+  --> examples/basic/0041-basic-block-value-reject.sx:10:10
+   |
+10 |     n * 2;
+   |          ^