P5.4 core: drive the whole build from sx default_pipeline (no auto-emit/link)

The compiler's post-IR role shrinks to: codegen -> invoke the build callback.
There is NO Zig auto-emit / auto-link anymore; emit + link are sx-called actions.

- emit_object() is now an ACTION (verify + emit via a host BuildHooks vtable),
  returning the object path. New query primitives build_output/build_target/
  build_frameworks/build_flags (data reads from the merged BuildConfig).
- library/modules/build.sx imports compiler.sx and defines default_pipeline:
  emit_object -> gather c_object_paths -> link(objs, output, libs, fws, flags,
  target). The std<->build import cycle is handled by the resolver.
- The compiler FORCE-LOWERS default_pipeline (well-known name) and AUTO-INVOKES
  it post-codegen when no on_build/set_post_link_callback override was
  registered (driver's final fallback: invokeByName default_pipeline).
- Prelude-less programs (e.g. asm tests) don't import build.sx, so the BUILD
  path auto-imports modules/build.sx (idempotent if already transitive) so
  default_pipeline is always available. JIT sx run is untouched (emits in-process).
- Removed the build cache short-circuits (incompatible with the always-run sx
  driver; a future cache can live in default_pipeline).

Benign 37-.ir churn (build.sx grew); zero behavior changes (verified diff is
.ir-only). 705/0 both gates.

src/ir/compiler_lib.zig

@@ -72,6 +72,10 @@ pub const bound_fns = [_]BoundFn{
     .{ .sx_name = "link_libraries", .handler = handleBuildPipelineQuery },
     .{ .sx_name = "emit_object", .handler = handleBuildPipelineQuery },
     .{ .sx_name = "link", .handler = handleBuildPipelineQuery },
+    .{ .sx_name = "build_output", .handler = handleBuildPipelineQuery },
+    .{ .sx_name = "build_target", .handler = handleBuildPipelineQuery },
+    .{ .sx_name = "build_frameworks", .handler = handleBuildPipelineQuery },
+    .{ .sx_name = "build_flags", .handler = handleBuildPipelineQuery },
 };
 
 /// Legacy-path stub for the Phase 5 build-pipeline primitives — see the