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/1189-diagnostics-unknown-builtin.stderr

@@ -0,0 +1,5 @@
+error: unknown #builtin 'mystery'
+  --> examples/diagnostics/1189-diagnostics-unknown-builtin.sx:12:1
+   |
+12 | mystery :: ($T: Type, x: T) -> T #builtin;
+   | ^^^^^^^