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/0734-modules-flat-same-name-ufcs-ambiguous/a.sx

@@ -0,0 +1,2 @@
+// a.sx authors `dup` (+1). One of two distinct flat authors of `dup`.
+dup :: ufcs (x: i64) -> i64 { return x + 1; }

examples/modules/0734-modules-flat-same-name-ufcs-ambiguous/b.sx

@@ -0,0 +1,3 @@
+// b.sx authors its OWN `dup` (+2) — the second distinct flat author. Main
+// imports both and authors neither, so `v.dup()` from main is ambiguous.
+dup :: ufcs (x: i64) -> i64 { return x + 2; }