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/1003-errors-raise-rejections.stderr

@@ -0,0 +1,17 @@
+error: error tag 'error.NotInSet' is not in error set 'ParseErr'
+  --> examples/errors/1003-errors-raise-rejections.sx:17:11
+   |
+17 |     raise error.NotInSet;            // error: NotInSet not in ParseErr
+   |           ^^^^^^^^^^^^^^
+
+error: error tag 'error.Weird' is not in caller's error set 'ParseErr'
+  --> examples/errors/1003-errors-raise-rejections.sx:24:5
+   |
+24 |     raise e;                         // error: OtherErr not subset of ParseErr
+   |     ^^^^^^^^
+
+error: `raise` is only valid inside a failable function (a return type with `!` or `!Named`)
+  --> examples/errors/1003-errors-raise-rejections.sx:30:5
+   |
+30 |     raise error.BadDigit;            // error: main (-> i32) is not failable
+   |     ^^^^^^^^^^^^^^^^^^^^^