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.

src/main.zig

@@ -824,7 +824,7 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
     // `set_post_link_module("name")`, re-enter the IR interpreter and
     // invoke that sx function now. A `false` return fails the build.
     if (comp.getPostLinkCallback()) |fid| {
-        const ret = comp.invokeByFuncId(fid, &.{}) catch |err| {
+        const ret = comp.invokeByFuncId(fid, comp.getPostLinkTakesOptions()) catch |err| {
             printInterpBailDiag(&comp, "post-link callback", err);
             return error.CompileError;
         };
@@ -835,7 +835,7 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
     } else if (comp.getPostLinkModule()) |mod_name| {
         const qualified = try std.fmt.allocPrint(allocator, "{s}.bundle_main", .{mod_name});
         defer allocator.free(qualified);
-        const ret_opt = comp.invokeByName(qualified, &.{}) catch |err| {
+        const ret_opt = comp.invokeByName(qualified, false) catch |err| {
             const label = try std.fmt.allocPrint(allocator, "post-link module '{s}.bundle_main'", .{mod_name});
             defer allocator.free(label);
             printInterpBailDiag(&comp, label, err);