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
  - --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)

Examples
```bash
# Emit JSON only
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
- The script uses Stage‑B entry (lang/src/compiler/entry/compiler_stageb.hako).
- 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.