P5.2 metadata queries: c_object_paths / link_libraries on the VM

Two abi(.compiler) build-pipeline primitives the sx driver will pass to link:
- c_object_paths() -> List(string)  (#import c companion objects)
- link_libraries() -> List(string)  (#library names)

They live in a new stdlib home library/modules/std/build.sx and are serviced
by comptime_vm.callCompilerFn reading two new BuildConfig fields that main.zig
forwards before the post-link callback. New reusable VM helper makeStringList
builds a List(string) in flat memory from the call's result type offsets
(target-aware); invoke/callCompilerFn now thread ins.ty for that. Legacy
handlers bail loudly (VM-only by nature — post-link; List(string) isn't
faithfully buildable in the legacy Value model, 0141).

Smoke test examples/1662-platform-build-pipeline-queries (AOT + a 1-line C
#source → one object): a post-link callback verifies the VM-built list is
well-formed; build exit 0 only if so (negative-probe confirmed a real guard).

emit_object + link (the actions) deferred to P5.2b — they replace the Zig
driver's auto-emit/auto-link and need a host-installed callback vtable.

703/0 both gates.

src/ir/compiler_hooks.zig

@@ -57,6 +57,15 @@ pub const BuildConfig = struct {
     /// sx bundler can switch on iOS vs. macOS vs. simulator.
     target_triple: ?[]const u8 = null,
 
+    /// C companion object files (`#import c { #source ... }`, compiled to `.o`)
+    /// and `#library` link names, forwarded by main.zig before the post-link
+    /// callback so the sx-driven build pipeline (Phase 5) can read them via the
+    /// `c_object_paths()` / `link_libraries()` compiler primitives and pass them
+    /// to `link`. Slices reference compiler-owned memory that outlives the
+    /// callback.
+    c_object_paths: []const []const u8 = &.{},
+    link_libraries: []const []const u8 = &.{},
+
     /// Frameworks the binary links against (`-framework` names) and
     /// the search paths to look them up in (`-F` directories), forwarded
     /// from the link step so the sx bundler can embed them into