test(asm): Phase 0.0 — corpus target-gating + .build JSON config

Adds per-example build/run directives to the corpus runner via an optional
`expected/<name>.build` JSON sidecar (`BuildConfig { aot, target }`), replacing
the standalone `.aot` marker. Threads `--target` into the run/build/ir spawns
and gates the execute path on host arch+os match; a cross-target example fails
loudly ("ir-only mode not yet implemented") pending Phase 0.1.

- corpus_run.test.zig: BuildConfig + std.json parse (unknown-key => error),
  hostMatchesTarget (shorthand-expand + arch/os token match, arm64->aarch64),
  withTarget argv helper; unit tests for both.
- migrate 1226/1227 `.aot` markers -> `.build` { "aot": true }.
- lock fixture 1638-platform-target-host (`.build` { "target": "macos" }).

Test-infra only; no compiler code. zig build test green (646 corpus, 444 unit).

current/CHECKPOINT-ASM.md

@@ -6,19 +6,46 @@ commit, one step at a time per the cadence rule (no commit may both add a test
 and make it pass).
 
 ## Last completed step
-None — plan authored, not yet started.
+**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}-*`.
 
 ## Current state
-Design fully converged (`docs/inline-asm-design.md`). Feasibility confirmed:
-`llvm_api.c.*` exposes `LLVMGetInlineAsm` / `LLVMBuildCall2` /
-`LLVMAppendModuleInlineAsm` (LLVM@19). No code written.
+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).
 
 ## Next step
-**A.0** — add the `kw_asm` keyword (`src/token.zig` Tag + `StaticStringMap`) and a
-unit lex test. Then A.1 (parse `asm { … }` → `AsmExpr`, lowering bails loudly).
+**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.
 
 ## Log
 - (init) Plan + design doc written; ASM stream opened.
+- (0.0) Corpus runner target-gating: `<name>.build` JSON config (replaces `.aot`
+  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.
 
 ## Known issues
 None yet.