P5.3: on_build(cb) build-callback registrar; callback takes BuildOptions

Per user design: on_build(build) is the build-callback registrar (a free fn),
generalizing set_post_link_callback — the callback is (opt: BuildOptions) ->
bool and the compiler invokes it post-codegen WITH the BuildOptions handle.

- VM: callCompilerFn 'on_build' arm + legacy handleOnBuild, both set
  post_link_callback_fn + a new BuildConfig.post_link_takes_options flag.
- comptime_vm: runEntry refactored to runEntryArgs(extra) (implicit ctx +
  explicit args); new public runBuildCallback(..., pass_options) passes the
  opaque BuildOptions handle (one word) after the ctx. The fat-config
  marshaling fear is moot — the handle is a single null-sentinel word.
- core.invokeByFuncId/invokeByName take pass_options (was an unused args
  slice); main.zig passes comp.getPostLinkTakesOptions().
- build.sx: on_build decl (set_post_link_callback kept for now).

Smoke test examples/1664-platform-on-build-callback (AOT): #run on_build(build)
with build :: (opt: BuildOptions) -> bool; the callback is invoked with the
handle arg (runEntryArgs param-count match) and runs the primitives.

Benign .ir churn (37 snapshots: type table +1 for the on_build fn type +
global renumber; behavior identical). 705/0 both gates.

examples/1664-platform-on-build-callback.sx

@@ -0,0 +1,18 @@
+#import "modules/std.sx";
+#import "modules/build.sx";
+#import "modules/compiler.sx";
+
+// P5.3 smoke test — `on_build(build)` registers the build callback (the Phase 5
+// form), and the compiler invokes it post-codegen WITH the BuildOptions handle
+// as an arg (`build :: (opt: BuildOptions) -> bool`). This exercises the VM
+// entry that passes an explicit arg after the implicit context (runEntryArgs).
+// The callback uses the build-pipeline primitives (no opt accessors yet) and
+// returns true; AOT observes success via the build exit code.
+
+build :: (opt: BuildOptions) -> bool abi(.compiler) {
+    obj := emit_object();
+    return obj.len > 0;
+}
+#run on_build(build);
+
+main :: () { print("runtime main\n"); }