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/1163-diagnostics-array-const-index-oob.sx

@@ -0,0 +1,12 @@
+// A compile-time index into an array constant is bounds-checked at fold
+// time — out of range is a diagnostic, never a wrap or a silent
+// runtime read.
+
+#import "modules/std.sx";
+
+K : [4]i64 : .[11, 22, 33, 44];
+
+main :: () {
+    b : [K[9]]u8 = ---;
+    print("{}\n", b.len);
+}