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/expected/0775-modules-qualified-generic-missing-member.stderr

@@ -0,0 +1,5 @@
+error: namespace 'a' has no member 'Box'
+  --> examples/modules/0775-modules-qualified-generic-missing-member.sx:20:9
+   |
+20 |     x : a.Box(i64) = .{ x = 1, y = 2 };
+   |         ^^^^^^^^^^