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/memory/expected/0801-memory-slices.stdout

@@ -0,0 +1,9 @@
+arr.len = 5
+arr[1..4] = [1, 4, 1]
+sub.len = 3
+arr[..3] = [3, 1, 4]
+arr[2..] = [4, 1, 5]
+sl[1..4] = [20, 30, 40]
+mid[1..] = [30, 40]
+msg[6..11] = world
+msg[..5] = hello