P5.1: post-link build driver runs on the comptime VM (no fallback)

core.invokeByFuncId routes the post-link callback through comptime_vm.tryEval
instead of the legacy Interpreter. REQUIRED because the sx build driver
allocates/grows Lists, which the legacy interp can't do at comptime (issue
0141: struct_get: base has no fields); the VM can. No fallback (a
side-effecting post-link callback can't double-execute): a VM bail is a hard
build error (comptime_vm.last_bail_reason, surfaced by printInterpBailDiag).
BuildConfig + import_sources threaded in; non-empty args rejected loudly.
flushInterpOutput deleted (VM out writes direct via host-FFI).

Smoke test examples/1661-platform-post-link-vm-list (AOT): a post-link
callback grows a List to 3 + returns len==3, so the build succeeds (exit 0)
only via the VM. First corpus coverage of the post-link path.

702/0 both gates.

examples/expected/1661-platform-post-link-vm-list.build

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

examples/expected/1661-platform-post-link-vm-list.exit

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

examples/expected/1661-platform-post-link-vm-list.stderr

@@ -0,0 +1 @@
+

examples/expected/1661-platform-post-link-vm-list.stdout

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