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/main.zig

@@ -717,6 +717,11 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
         if (merged_config.triple) |t| e.build_config.target_triple = std.mem.span(t);
         e.build_config.target_frameworks = fws;
         e.build_config.target_framework_paths = merged_config.framework_paths;
+        // Phase 5: the sx-driven build pipeline reads these via the
+        // `c_object_paths()` / `link_libraries()` compiler primitives. Both
+        // slices reference compileWithTimer locals that outlive the callback.
+        e.build_config.c_object_paths = c_obj_paths;
+        e.build_config.link_libraries = libs;
         // Android-specific bundling state.
         if (e.build_config.manifest_path == null) e.build_config.manifest_path = merged_config.manifest_path;
         if (e.build_config.keystore_path == null) e.build_config.keystore_path = merged_config.keystore_path;