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/platform/expected/1648-platform-asm-global.ir

@@ -0,0 +1,16 @@
+
+module asm ".global _my_add"
+module asm "_my_add:"
+module asm "    add x0, x0, x1"
+module asm "    ret"
+
+; Function Attrs: nounwind
+declare i64 @my_add(i64, i64) #0
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  %call = call i64 @my_add(i64 40, i64 2)
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}