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.
