Source Extensions Policy — .nyash vs .hako (Interim)

Intent
- Keep development stable while Hakorune VM (v1 Dispatcher/IR/φ) is brought up.
- Avoid cross‑contamination between frontends; converge at MIR.

Execution Mapping (current)
- .nyash → Nyash VM (Rust): NyashParser → MIR → VM. Full runtime/plugins path.
- .hako  → Hakorune VM (v1): JSON v1 Dispatcher/IR/φ（bring‑up coverage; guarded extern）.
- verify (MIR v1) → Hakorune primary, Core fallback for diagnosis.

Resolver/Include/Normalize
- Using: unify to text‑merge (merge_prelude_text). AST prelude merge is retired.
- Include: language‑level unsupported; quick profile treats include as ERROR. Preinclude is test‑harness only（verify の include fallback は撤去済み）。
- Normalize (inline/dev): CRLF→LF, redundant `; }` trimmed, tolerant `local` at line head in Hakorune inline drivers.

Fail‑Fast Guards
- Hako in Nyash VM: rejected by default (Fail‑Fast). Toggle: `HAKO_FAIL_FAST_ON_HAKO_IN_NYASH_VM=0` for dev only.
- Extern (Hako provider): `HAKO_V1_EXTERN_PROVIDER=1`; optional C‑ABI tag/bridge with `HAKO_V1_EXTERN_PROVIDER_C_ABI=1` (default OFF).

Why two extensions now?
- Same language intent, but different stability constraints at the frontend boundary:
  - .hako through NyashParser caused parse/timeout/alias issues historically.
  - We intentionally stopped that path and direct .hako to Hakorune VM while it matures.

Migration Plan (phased)
1) P2: IR iteration complete; φ table robust; dispatcher loop scan‑free; extern canaries PASS without harness shim.
2) P3: verify default → Hakorune (Core fallback); document toggles and remove ad‑hoc heuristics.
3) Prep: introduce stable Nyash→MIR v1 emit route to feed Hakorune VM when (and if) we want .nyash on hv1.
4) Deprecation: warn on `.nyash` (opt‑in), then Fail‑Fast once hv1 parity is sufficient; remove legacy code in a later phase.

Best Practices (now)
- Prefer alias/modules over path using; avoid include in source.
- Keep quotes ASCII (`"`); avoid trailing semicolons before `}`.
- For verify, pass JSON via env (`NYASH_VERIFY_JSON`) and parse last numeric line as rc.
 - Inline（-c）ドライバは alias のみで構成（include 不要）。