docs/runtime: externcall + println normalization; NyRT silent result flag; PyVM console warn/error/trace routing\nselfhost(M2): add MirEmitterBox (Return(Int)->const+ret) and --emit-mir switch; initial selfhost smoke\nrunner: child args/env hardening for selfhost (NYASH_NY_COMPILER_CHILD_ARGS under --; NYASH_JSON_ONLY=1, NYASH_VM_USE_PY=1, NYASH_DISABLE_PLUGINS=1); VM path strips using lines when NYASH_ENABLE_USING=1\nmeta: clarify PyVM/llvmlite as primary (vm/interpreter legacy OFF) in CURRENT_TASK.md; README links to externcall docs

docs/reference/runtime/externcall.md

@@ -0,0 +1,42 @@
+# ExternCall — Runtime Interfaces (env.*)
+
+Overview
+- ExternCall represents calls to host-provided interfaces, addressed by an interface name and method, e.g. `env.console.log`.
+- In Nyash, source-level `print/println` normalize to an ExternCall targeting the console interface.
+
+Normalization of println/print
+- Builder/Optimizer rewrites language-level printing to ExternCall:
+  - `println(x)` or `ConsoleBox.println(x)` → `ExternCall("env.console", "log", [x])`
+- Rationale: unifies backends and keeps ConsoleBox for other roles (vtable optimization, advanced console APIs).
+
+Backend Behavior
+- LLVM/AOT (EXE-first):
+  - `env.console.log` lowers to NyRT exports and links statically.
+  - Primary mapping uses pointer-API when possible to avoid handle churn:
+    - `nyash.console.log(i8*) -> i64`
+    - Fallback to handle-API helpers if only a handle is available.
+  - Runtime result line: NyRT prints `Result: <code>` after `ny_main()` returns. Set `NYASH_NYRT_SILENT_RESULT=1` to suppress for tests.
+- PyVM:
+  - Accepts `env.console.log/warn/error` and writes to stdout (MVP). Return is `0` when a destination is present.
+- JIT:
+  - Host-bridge directly prints the first argument to stdout for `env.console.log` minimal parity.
+
+MIR JSON v0 Encoding
+- Instruction shape:
+  - `{ "op": "externcall", "func": "env.console.log", "args": [<vid>], "dst": <vid|null>, "dst_type": "i64"? }`
+  - Builder may also emit `"func": "nyash.console.log"` in some paths; both are accepted by backends.
+
+Key Fields (JSON v0, minimal)
+- `op`: literal `"externcall"`.
+- `func`: fully qualified name. Preferred: `"env.console.log"`. Accepted: `"nyash.console.log"`.
+- `args`: value-ids array (each is an integer referencing a producer).
+- `dst`: optional value-id to store result; may be null when ignored.
+- `dst_type` (optional): backend hint. For console methods, `"i64"` is used when `dst` is present.
+
+
+Return Value Convention
+- Console methods return `i64` status (typically 0). Most user code ignores it; when `dst` is set, backends materialize `0`.
+
+Guidance
+- Use `print/println` in Nyash source; the toolchain normalizes to ExternCall.
+- Prefer exit code assertions in EXE-first tests. If you must compare stdout, set `NYASH_NYRT_SILENT_RESULT=1` to hide NyRT's `Result:` line.