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/0725-modules-flat-dir-same-name/d1/one.sx

@@ -0,0 +1,3 @@
+// d1's author of `tag`. `caller1` (also in d1) binds d1's own `tag` (100).
+tag     :: () -> i64 { return 100; }
+caller1 :: () -> i64 { return tag(); }

examples/modules/0725-modules-flat-dir-same-name/d2/two.sx

@@ -0,0 +1,4 @@
+// d2's author of `tag`. `caller2` (also in d2) binds d2's own `tag` (200),
+// even though d1's `tag` is the first-wins merge winner.
+tag     :: () -> i64 { return 200; }
+caller2 :: () -> i64 { return tag(); }