Phase 3: Move host files to hosts/python/ and hosts/javascript/

Python: bootstrap.py, platform.py, transpiler.sx, boundary_parser.py, tests/
JavaScript: bootstrap.py, cli.py, platform.py, transpiler.sx
Both bootstrappers verified — build from new locations, output to shared/.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/javascript/cli.py

@@ -0,0 +1,66 @@
+#!/usr/bin/env python3
+"""
+Bootstrap compiler: reference SX evaluator → JavaScript.
+
+This is now a thin shim that delegates to run_js_sx.py (the self-hosting
+bootstrapper). The hand-written JSEmitter has been replaced by js.sx.
+
+Usage:
+    python bootstrap_js.py
+"""
+from __future__ import annotations
+
+import os
+import sys
+
+_HERE = os.path.dirname(os.path.abspath(__file__))
+_PROJECT = os.path.abspath(os.path.join(_HERE, "..", "..", ".."))
+if _PROJECT not in sys.path:
+    sys.path.insert(0, _PROJECT)
+
+# Re-export everything that consumers import from this module.
+# Canonical source is now run_js_sx.py (self-hosting via js.sx) and platform_js.py.
+import sys, os
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "..")))
+from hosts.javascript.bootstrap import compile_ref_to_js, load_js_sx  # noqa: F401
+from hosts.javascript.platform import (  # noqa: F401
+    extract_defines,
+    ADAPTER_FILES, ADAPTER_DEPS, SPEC_MODULES, EXTENSION_NAMES,
+    PREAMBLE, PLATFORM_JS_PRE, PLATFORM_JS_POST,
+    PRIMITIVES_JS_MODULES, _ALL_JS_MODULES, _assemble_primitives_js,
+    PLATFORM_DEPS_JS, PLATFORM_PARSER_JS, PLATFORM_DOM_JS,
+    PLATFORM_ENGINE_PURE_JS, PLATFORM_ORCHESTRATION_JS, PLATFORM_BOOT_JS,
+    CONTINUATIONS_JS, ASYNC_IO_JS,
+    fixups_js, public_api_js, EPILOGUE,
+)
+
+
+if __name__ == "__main__":
+    import argparse
+    p = argparse.ArgumentParser(description="Bootstrap-compile SX reference spec to JavaScript")
+    p.add_argument("--adapters", "-a",
+                    help="Comma-separated adapter list (html,sx,dom,engine). Default: all")
+    p.add_argument("--modules", "-m",
+                    help="Comma-separated primitive modules (core.* always included). Default: all")
+    p.add_argument("--extensions",
+                    help="Comma-separated extensions (continuations). Default: none.")
+    p.add_argument("--spec-modules",
+                    help="Comma-separated spec modules (deps). Default: none.")
+    default_output = os.path.join(_HERE, "..", "..", "shared", "static", "scripts", "sx-browser.js")
+    p.add_argument("--output", "-o", default=default_output,
+                    help="Output file (default: shared/static/scripts/sx-browser.js)")
+    args = p.parse_args()
+
+    adapters = args.adapters.split(",") if args.adapters else None
+    modules = args.modules.split(",") if args.modules else None
+    extensions = args.extensions.split(",") if args.extensions else None
+    spec_modules = args.spec_modules.split(",") if args.spec_modules else None
+    js = compile_ref_to_js(adapters, modules, extensions, spec_modules)
+
+    with open(args.output, "w") as f:
+        f.write(js)
+    included = ", ".join(adapters) if adapters else "all"
+    mods = ", ".join(modules) if modules else "all"
+    ext_label = ", ".join(extensions) if extensions else "none"
+    print(f"Wrote {args.output} ({len(js)} bytes, adapters: {included}, modules: {mods}, extensions: {ext_label})",
+          file=sys.stderr)