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/protocols/0821-protocols-same-name-method-ambiguous/a.sx

@@ -0,0 +1,3 @@
+// One of two flat-imported same-name `Box` authors — the bare `Box` in the
+// protocol return is a genuine collision the source cannot disambiguate.
+Box :: struct { a: i32; }

examples/protocols/0821-protocols-same-name-method-ambiguous/b.sx

@@ -0,0 +1,3 @@
+// The second flat-imported same-name `Box` author. Two distinct flat authors and
+// no own author → the protocol return `Box` is ambiguous.
+Box :: struct { b: i32; }