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/1641-platform-asm-missing-volatile.sx

@@ -0,0 +1,6 @@
+// ASM stream Phase B — an asm with no value outputs yields no result, so its
+// effects could be deleted unless it is marked `volatile`. This omits
+// `volatile` ⇒ a compile error. Pins that diagnostic (mirrors Zig's rule).
+// Called from `main` so lowering reaches the asm body.
+nope :: () { asm { "nop" }; }
+main :: () { nope(); }