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/modules/0791-modules-same-name-const-multi-level-cross-module/b.sx

@@ -0,0 +1,4 @@
+// Module B authors a full same-name shadow `M :: 10`, `K :: M + 1` (= 11). `main`
+// flat-imports B, so bare `K` in `main` is B's (= 11).
+M :: 10;
+K :: M + 1;