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.

library/modules/build.sx

@@ -1,3 +1,9 @@
+// `List` (for the default build script) + the compiler-API build primitives
+// (`emit_object`/`link`/`c_object_paths`/…). The std↔build import cycle (std's
+// tail imports cli.sx which imports this file) is handled by the resolver.
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+
 OperatingSystem :: enum { macos; linux; windows; wasm; ios; android; unknown; }
 Architecture :: enum { aarch64; x86_64; wasm32; wasm64; unknown; }
 
@@ -102,3 +108,21 @@ set_post_link_callback :: ufcs (self: BuildOptions, cb: () -> bool) abi(.compile
 // `build :: (opt: BuildOptions) -> bool abi(.compiler) { … }`. The stdlib default
 // implementation lives below (the default `build` script).
 on_build :: (cb: (opt: BuildOptions) -> bool abi(.compiler)) abi(.compiler);
+
+// ── The default build script ────────────────────────────────────────────────
+//
+// `default_pipeline` is the stdlib build driver: the compiler invokes it after
+// codegen (everything is sx-driven — there is no auto-emit/auto-link). It emits
+// the sx object, gathers the C companion objects, and links them into the output
+// with the build's libraries / frameworks / flags / target. A user overrides the
+// whole pipeline with their own `#run on_build(custom);` in main.sx (last-wins).
+// The compiler FORCE-LOWERS this well-known name and auto-invokes it after
+// codegen when no `#run on_build(custom);` override was registered (no library
+// `#run` needed). A user override takes over entirely.
+default_pipeline :: (opt: BuildOptions) -> bool abi(.compiler) {
+    obj := emit_object();
+    objs := c_object_paths();
+    objs.append(obj);
+    link(objs, build_output(), link_libraries(), build_frameworks(), build_flags(), build_target());
+    return true;
+}

library/modules/compiler.sx

@@ -15,11 +15,17 @@
 c_object_paths :: () -> List(string) abi(.compiler);
 link_libraries :: () -> List(string) abi(.compiler);
 
-// The object file the compiler emitted for this build. The compiler emits it
-// eagerly; this returns its path (a query, not an action). The sx driver passes
-// it to `link` alongside the C objects.
+// Verify + emit the codegen'd module to its object file; returns the path. An
+// ACTION — the compiler no longer auto-emits; the sx driver calls this.
 emit_object :: () -> string abi(.compiler);
 
+// Build-config metadata the sx driver passes to `link` (the merged CLI + `#run`
+// build config the compiler accumulated for this build).
+build_output     :: () -> string abi(.compiler);        // the output binary path
+build_target     :: () -> string abi(.compiler);        // the target triple ("" = host)
+build_frameworks :: () -> List(string) abi(.compiler);  // `-framework` names
+build_flags      :: () -> List(string) abi(.compiler);  // extra link flags
+
 // Link `objects` into `output`, with the given libraries / frameworks / link
 // flags / target triple. The one genuine ACTION primitive — the compiler keeps
 // the proven linker (Option B); the sx driver orchestrates. Not fallible (the