P5.4: migrate all callers to on_build; delete set_post_link_callback

on_build is now the sole post-build callback mechanism. Migrated the 9 callers
(0602/0603/1611/1614/1615/1616 + the platform bundle_main) from
opts.set_post_link_callback(cb) to on_build(cb), giving each callback the
(opt: BuildOptions) param. Deleted set_post_link_callback from build.sx,
compiler_lib (bound_fns + handleSetPostLinkCallback), and the VM arm.

Reworked the P5 smoke tests for the new semantics: an on_build override REPLACES
the build (must emit+link or delegate), unlike the old post-link callback which
ran after the auto-link. 1662 (queries) + 1664 (override+List-grow) now delegate
to default_pipeline for the real build; deleted 1661/1663 (the primitives are now
exercised by every AOT build). bundle_main invoked with pass_options=true.

Benign 37-.ir churn (build.sx shrank). 703/0 both gates.

library/modules/platform/bundle.sx

@@ -29,7 +29,7 @@
 // `abi(.compiler)` so the backend doesn't lower it and its `build_options()`
 // compiler-API call is permitted (it would otherwise be rejected as a
 // comptime-only function called at runtime).
-bundle_main :: () -> bool abi(.compiler) {
+bundle_main :: (opt: BuildOptions) -> bool abi(.compiler) {
     opts := build_options();
     binary := opts.binary_path();
     bundle := opts.bundle_path();