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/modules/0764-modules-import-generic-head-non-transitive/b.sx

@@ -0,0 +1,8 @@
+#import "c.sx";
+
+// b.sx directly imports c.sx, so it CAN instantiate `Box(i64)` — proving the
+// generic head is only one flat hop away here, two hops from a file that
+// imports b.sx.
+b_make :: () -> Box(i64) {
+    .{ v = 99 }
+}

examples/modules/0764-modules-import-generic-head-non-transitive/c.sx

@@ -0,0 +1 @@
+Box :: struct($T: Type) { v: T; }