Hybrid Selfhost Build (80/20)

Purpose
- Provide a minimal, fast path to compile Hako source via Hakorune Stage‑B to Program(JSON v0), and optionally run it via Core‑Direct (in‑proc).
- Future: add MIR emit and ny-llvmc EXE build in small increments.

Script
- tools/selfhost/selfhost_build.sh
  - --in <file.hako>: input Hako source
  - --json <out.json>: write Program(JSON v0); default: /tmp/hako_stageb_$$.json
  - --mir <out.json>: emit MIR(JSON) from source (runner path)
  - --exe <out>: build native executable via ny-llvmc (llvmlite harness)
  - --run: run via Gate‑C/Core Direct (in‑proc). Exit code mirrors program return.
  - Env:
    - NYASH_BIN: path to hakorune/nyash binary (auto-detected)
    - NYASH_ROOT: repo root (auto-detected)
    - HAKO_USE_BUILDBOX=1: use BuildBox for emit-only (no run/exe)

Examples
```bash
# Emit JSON only (Stage‑B)
tools/selfhost/selfhost_build.sh --in apps/demo/main.hako --json /tmp/demo.json

# Run and use exit code
tools/selfhost/selfhost_build.sh --in apps/demo/return7.hako --run; echo $?
```

Notes
- Stage‑B emit uses either the Stage‑B entry or BuildBox（HAKO_USE_BUILDBOX=1 for emit-only）
- Runner executes Core‑Direct in-proc under HAKO_CORE_DIRECT_INPROC=1.
- For heavier cases (bundles/alias/require), keep Stage‑B canaries opt‑in in quick profile.