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/1175-diagnostics-extern-export-conflict.sx

@@ -0,0 +1,11 @@
+// Phase 4 (FFI-linkage) interplay diagnostic: `extern` and `export` are the two
+// values of the same linkage axis — a declaration is either an import (`extern`)
+// or a definition (`export`), never both. The parser rejects the redundant
+// second keyword with a clear message (instead of the bare "expected ';'" the
+// body parser would otherwise emit).
+//
+// Expected: one error caret on the second keyword; exit 1.
+
+f :: (a: i32) -> i32 extern export;
+
+main :: () -> i32 { 0 }