BuildBox: env alias/require minimal bundling; selfhost tool: add --mir and docs

- BuildBox.emit_program_json_v0: use BundleResolver via env (HAKO_BUNDLE_ALIAS_TABLE, HAKO_REQUIRE_MODS)
- selfhost_build.sh: add --mir to emit MIR(JSON); document HAKO_USE_BUILDBOX for emit-only path

tools/selfhost/README.md

@@ -8,14 +8,17 @@ 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
+# Emit JSON only (Stage‑B)
 tools/selfhost/selfhost_build.sh --in apps/demo/main.hako --json /tmp/demo.json
 
 # Run and use exit code
@@ -23,7 +26,6 @@ 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).
+- 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.
-