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/1639-platform-target-cross.sx

@@ -0,0 +1,7 @@
+// Phase 0 (ASM stream) test-infra lock: exercises the corpus runner's
+// CROSS-TARGET ir-only path. The `.build` pins `x86_64-linux`, which does NOT
+// match this aarch64 host, so the runner skips run/build/exec and verifies via
+// `sx ir --target x86_64-linux` only — asserting exit + the `.ir` snapshot +
+// stderr (no `.stdout`). Asm-free on purpose: it locks the harness gating, not
+// any inline-asm lowering (that arrives in Phase A+).
+main :: () -> i64 { return 0; }