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/1155-diagnostics-for-not-iterable.sx

@@ -0,0 +1,12 @@
+// The collision case of the positional capture rule: `for f(n) { }` reads
+// `(n)` as the capture, making the iterable `f` itself — not iterable, with
+// the parenthesize/add-capture hint.
+
+#import "modules/std.sx";
+
+f :: (n: i64) -> i64 { n }
+
+main :: () {
+    n := 1;
+    for f(n) { }
+}