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/errors/expected/1017-errors-onfail-reject.stderr

@@ -0,0 +1,5 @@
+error: `onfail` is only valid inside a failable function (a return type with `!` or `!Named`) — use `defer` for unconditional cleanup
+  --> examples/errors/1017-errors-onfail-reject.sx:9:5
+   |
+ 9 |     onfail print("never fires\n");   // error: onfail outside a failable function
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^