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).

current/CHECKPOINT-ASM.md

@@ -6,39 +6,34 @@ commit, one step at a time per the cadence rule (no commit may both add a test
 and make it pass).
 
 ## Last completed step
-**0.0** — corpus runner target-gating + `<name>.build` JSON config. Added
-`BuildConfig` (`std.json.parseFromSliceLeaky` → `struct { aot, target }`,
-unknown-key ⇒ `error.UnknownField`) replacing the standalone `.aot` marker;
-migrated the 2 existing `.aot` markers (1226/1227) to `{ "aot": true }`; threaded
-`--target` into the `run`/`build`/`ir` spawns via `withTarget`; added
-`hostMatchesTarget` (shorthand-expand + arch/os token match, `arm64`→`aarch64`)
-gating the execute path. Cross-target mismatch fails **loudly** (placeholder until
-0.1's ir-only branch) — verified the bail fires (`target=linux,
-host=aarch64-macos`). Locked with `examples/1638-platform-target-host.sx`
-(`.build` `{ "target": "macos" }`, runs natively + asserts stdout) + unit tests
-for the JSON parse and `hostMatchesTarget`. `zig build test` green (646 corpus, 0
-failed; 444 unit, 0 failed). Files: `src/corpus_run.test.zig`,
-`examples/1638-*`, `examples/expected/{1226,1227,1638}-*`.
+**0.1** — corpus runner **ir-only branch** for cross-target examples. Replaced
+0.0's loud placeholder bail: when `cfg.target` doesn't match the host (`ir_only`),
+`sweepRoot` skips run/build/exec and verifies via `sx ir --target` only —
+asserting `.exit` (ir cmd) + `.ir` (normalized stdout) + `.stderr`, never
+`.stdout` (write skipped in update mode, assertion skipped in verify mode). An
+`.ir` snapshot is **required** in ir-only mode — its absence is a loud failure
+("needs an .ir snapshot for ir-only mode"). Locked with
+`examples/1639-platform-target-cross.sx` (asm-free `main :: () -> i64 { return 0;
+}`), `.build` `{ "target": "x86_64-linux" }`, + checked-in `.ir`. Verified both
+guards fire: corrupting the `.ir` → IR mismatch; deleting it → the require-failure.
+`zig build test` green (647 corpus, 0 failed; 444 unit). Files:
+`src/corpus_run.test.zig`, `examples/1639-*`.
 
 ## Current state
-Phase 0 step 0.0 landed (test-infra only — no compiler code touched). The corpus
-runner now reads `expected/<name>.build` and threads/gates `--target`; an
-arch-pinned example whose target matches the host **executes**, a mismatch
-currently **bails loudly** ("cross-target ir-only mode not yet implemented"). Phase
-A–E feasibility already confirmed against the live tree (`LLVMGetInlineAsm` /
-`LLVMBuildCall2` / `LLVMAppendModuleInlineAsm` in LLVM@19 `Core.h`; ERR-stream
-`extractvalue`→tuple machinery in `emit_llvm.zig:726-927`; lib-less `extern`, 60
-sites; `--target` a global CLI flag).
+Phase 0 steps 0.0 + 0.1 landed (test-infra only — no compiler code). The corpus
+runner reads `expected/<name>.build` (JSON `{ aot, target }`), threads `--target`,
+and gates on host arch+os: a matching target **executes** (full exit/stdout/stderr
++ optional `.ir`); a mismatch runs **ir-only** (`sx ir --target`, asserting
+exit+ir+stderr, `.ir` required). Phase A–E feasibility already confirmed against
+the live tree (`LLVMGetInlineAsm` / `LLVMBuildCall2` / `LLVMAppendModuleInlineAsm`
+in LLVM@19 `Core.h`; ERR-stream `extractvalue`→tuple in `emit_llvm.zig:726-927`;
+lib-less `extern`, 60 sites; `--target` a global CLI flag).
 
 ## Next step
-**0.1** — implement the **mismatch ⇒ ir-only** branch in `sweepRoot` (replace the
-loud placeholder bail): when `cfg.target` doesn't match the host, skip
-run/build/exec and assert only `.exit`+`.ir`+`.stderr` from `sx ir --target`;
-require an `.ir` snapshot (loud failure if absent). Lock with
-`examples/16xx-platform-target-cross.sx` (asm-free `() -> i64 { return 0; }`),
-`.build` `{ "target": "x86_64-linux" }`, + a checked-in `.ir` snapshot. Then 0.2
-(CLAUDE.md §Testing/§Test-layout docs for `.build`), then Phase A (`kw_asm`). See
-`PLAN-ASM.md` Phase 0.
+**0.2** — docs: update CLAUDE.md §"Testing"/§"Test layout" to document the
+`<name>.build` JSON config (`aot` + `target` keys, ir-only gating) replacing the
+standalone `.aot` marker prose (~lines 435, 492). Then Phase A (`kw_asm` keyword +
+lex test). See `PLAN-ASM.md` Phase 0 / Phase A.
 
 ## Log
 - (init) Plan + design doc written; ASM stream opened.
@@ -46,6 +41,9 @@ require an `.ir` snapshot (loud failure if absent). Lock with
   marker), `--target` threading, `hostMatchesTarget` execute-gate, loud
   cross-target placeholder bail. Migrated 1226/1227 `.aot`→`.build`; locked with
   1638 fixture + unit tests. `zig build test` green.
+- (0.1) ir-only branch: cross-target examples verify via `sx ir --target` only
+  (exit+ir+stderr, no stdout; `.ir` required). Locked with 1639 fixture; verified
+  corrupt-.ir → mismatch and missing-.ir → loud failure. `zig build test` green.
 
 ## Known issues
 None yet.