P5.2b: link() build-pipeline action on the VM via a host vtable

The one genuine action primitive: link(objects, output, libraries, frameworks,
flags, target) in library/modules/std/build.sx. Per the user decision to drop
fallibility from the build callback, link is plain VOID — a link failure bails
on the VM (hard build error), no -> ! / failable-tuple needed.

comptime_vm.zig can't depend on the driver (core/main/target), so link
dispatches through a new compiler_hooks.BuildHooks { ctx, link } vtable that
main.zig installs into BuildConfig.build_hooks before the post-link callback.
The driver side is main.LinkHooksCtx (unions explicit + CLI link flags, calls
target.link). New VM readers readStringList / readStringArg (inverse of
makeStringList) decode the List(string)/string args from flat memory.

Smoke test examples/1663-platform-build-pipeline-link (AOT): a post-link
callback re-links the build's own objects (c_object_paths + emit_object) into a
temp output via sx link — the relinked binary is a functional executable that
runs. Negative-probe verified (bad path -> ld fails -> ComptimeVmBail -> build
exit 1). The Zig driver still auto-links; removing that is P5.4.

704/0 both gates.

examples/expected/1663-platform-build-pipeline-link.build

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

examples/expected/1663-platform-build-pipeline-link.exit

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

examples/expected/1663-platform-build-pipeline-link.stderr

@@ -0,0 +1 @@
+

examples/expected/1663-platform-build-pipeline-link.stdout

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