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/atomics/expected/1701-atomics-rmw.stdout

@@ -0,0 +1,13 @@
+old add: 10
+old sub: 15
+now: 12
+old and: 240
+old or:  48
+old xor: 51
+now: 60
+old min: 20
+old max: 8
+now: 15
+runtime signed max(-5,3): 3
+runtime signed min(-5,3): -5
+comptime signed max(-5,3)=3 min(-5,3)=-5