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"); }

examples/expected/1202-ffi-cc-c-large-aggregate.ir

@@ -668,6 +668,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1309-ffi-objc-class-method-lowering.ir

@@ -1170,6 +1170,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1314-ffi-objc-class-dealloc-roundtrip.ir

@@ -1186,6 +1186,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1329-ffi-objc-call-03-selector-sharing.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1347-ffi-objc-dsl-07-mangling-table.ir

@@ -1188,6 +1188,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1402-ffi-jni-call-03-methodid-sharing.ir

@@ -1164,6 +1164,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1403-ffi-jni-call-04-jint-return.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1404-ffi-jni-call-05-jlong-return.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1405-ffi-jni-call-06-jdouble-return.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1406-ffi-jni-call-07-jboolean-return.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1407-ffi-jni-call-08-jobject-return.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1408-ffi-jni-call-09-static.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1418-ffi-jni-class-08-call.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1421-ffi-jni-env-02-lexical-direct.ir

@@ -1162,6 +1162,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

examples/expected/1425-ffi-jni-main-03-ctor.ir

@@ -675,6 +675,9 @@ declare i64 @build_options() #0
 ; Function Attrs: nounwind
 declare void @set_post_link_callback(i64, ptr) #0
 
+; Function Attrs: nounwind
+declare void @on_build(ptr) #0
+
 ; Function Attrs: nounwind
 declare ptr @spaces.44(ptr, i32) #0
 

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

@@ -0,0 +1 @@
+{ "aot": true }

examples/expected/1664-platform-on-build-callback.exit

@@ -0,0 +1 @@
+0

examples/expected/1664-platform-on-build-callback.stderr

@@ -0,0 +1 @@
+

examples/expected/1664-platform-on-build-callback.stdout

@@ -0,0 +1 @@
+runtime main