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/basic/expected/0051-basic-for-range-bounds.stdout

@@ -0,0 +1,15 @@
+1 2 3 4 | 0<..<5
+0 1 2 3 4 5 | 0=..=5
+1 2 3 4 5 | 0<..=5
+0 1 2 3 4 | 0=..<5
+0 1 2 3 4 | 0..<5
+0 1 2 3 4 5 | 0..=5
+10@3 20@4 30@5 | xs, 2<..
+10@5 20@6 30@7 | xs, 5=..
+inline 0<..=3 sum=6
+2 3 4 | lo<..=hi
+x+2..=42: n=39 sum=897
+x+2<..<x*21: n2=37
+0..x*3: n3=6
+cmp ok
+shl=6