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/diagnostics/expected/1133-diagnostics-array-dim-negative-float.stderr

@@ -0,0 +1,5 @@
+error: array dimension must be non-negative, got -2
+  --> examples/diagnostics/1133-diagnostics-array-dim-negative-float.sx:10:10
+   |
+10 |     a : [-2.0]i64 = ---;
+   |          ^^^^