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/1133-diagnostics-array-dim-negative-float.sx

@@ -0,0 +1,12 @@
+// A NEGATIVE integral float (`-2.0`) used as an array dimension is a hard error.
+// The integral-float rule folds and negates it to `-2`, then the shared u32 dim
+// gate rejects a below-minimum dimension — a clean diagnostic + non-zero exit.
+//
+// Regression (F0.4 attempt 8, Agra ruling): integral floats fold, but a negative
+// result is still rejected (a dimension must be non-negative).
+#import "modules/std.sx";
+
+main :: () {
+    a : [-2.0]i64 = ---;
+    print("unreachable: {}\n", a.len);
+}