test(asm): Phase 0.1 — corpus ir-only branch for cross-target examples

When a `.build` target doesn't match the host, the runner can't execute the
example here, so it verifies via `sx ir --target` only: asserts exit + the `.ir`
snapshot (stdout) + diagnostics (stderr), never `.stdout`. An `.ir` snapshot is
REQUIRED in ir-only mode — its absence is a loud failure, never a silent pass.

- corpus_run.test.zig: ir_only flag (target set & !hostMatchesTarget); first
  dispatch arm runs `sx ir`, sets act_exit/act_err/act_ir; skip stdout in both
  update and verify modes; require ir_raw.
- lock fixture 1639-platform-target-cross (asm-free main, target x86_64-linux,
  checked-in .ir). Verified: corrupt .ir => IR mismatch; delete .ir => require
  failure.

Test-infra only; no compiler code. zig build test green (647 corpus, 444 unit).

examples/expected/1639-platform-target-cross.build

@@ -0,0 +1 @@
+{ "target": "x86_64-linux" }

examples/expected/1639-platform-target-cross.exit

@@ -0,0 +1 @@
+0

examples/expected/1639-platform-target-cross.ir

@@ -0,0 +1,6 @@
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  ret i32 0
+}

examples/expected/1639-platform-target-cross.stderr

@@ -0,0 +1 @@
+