Phase 20.34 prep: add using aliases in nyash.toml for hako.mir.builder & hako.llvm.emit; add README contracts for MirBuilderBox/LLVMEmitBox; verify canaries green

lang/src/mir/builder/README.md

@@ -0,0 +1,22 @@
+# MirBuilderBox — Program(JSON v0) → MIR(JSON v0)
+
+Responsibility
+- Convert Stage‑B Program(JSON v0) into MIR(JSON v0) for VM/LLVM lines.
+- Phase 20.34 starts with a delegate implementation to Runner and progressively internalizes ops.
+
+Interface (stable)
+- `emit_from_program_json_v0(program_json: String, opts: Map|Null) -> String|Null`
+  - Returns canonical MIR(JSON v0) on success; returns null and prints a tagged diagnostic on failure.
+
+Tags (Fail‑Fast, stable)
+- `[mirbuilder/input/null]` — input is null
+- `[mirbuilder/input/invalid]` — header missing (version/kind)
+- `[mirbuilder/delegate]` — delegate path selected (Runner `--program-json-to-mir`)
+- `[mirbuilder/delegate/missing]` — delegate/provider not wired yet
+
+Toggles (default OFF)
+- `HAKO_MIR_BUILDER_DELEGATE=1`: Use Runner `--program-json-to-mir` as a temporary provider
+
+Notes
+- Box‑First policy: boundary/contract first, then implementation. Keep tags stable; no silent fallback.
+- Large payloads: implementation may stream/json-scan later; initial version is string‑based.