json-native: enable float roundtrip in parser (NUMBER => float by '.'/exp); expand roundtrip smoke with more numeric cases

docs/reference/plugin-system/externcall.md

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+# ExternCall Policy (Phase 15)
+
+## Allowed Interfaces (minimal set)
+- `env.console.{log,warn,error,readLine}`
+- `env.debug.trace`
+- `env.system.{exit,now}` (if present)
+
+All other host interactions should go through BoxCall (NyRT or plugins).
+
+## Argument‑type‑based selection
+- For `env.console.{log,warn,error}` and `env.debug.trace`:
+  - If the single argument is `i8*` (C string), call the C‑string variant:
+    - `nyash.console.log(i8*)`, `nyash.console.warn(i8*)`, `nyash.console.error(i8*)`
+    - `nyash.debug.trace(i8*)`
+  - Otherwise convert to `i64` and call the handle variant:
+    - `nyash.console.log_handle(i64)`, `nyash.console.warn_handle(i64)`, `nyash.console.error_handle(i64)`
+    - `nyash.debug.trace_handle(i64)`
+
+## Rationale
+- Keeps the AOT string path fast and avoids accidental `inttoptr` of handles.
+- Avoids adding broad implicit conversions in ExternCall; selection is local and explicit.
+
+## Non‑LLVM Backends
+- VM, Cranelift JIT/AOT, and the interpreter may not implement this policy yet (not MIR14‑ready). LLVM is authoritative; other backends will align after stabilization.
+