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.

examples/1662-platform-build-pipeline-queries.c

@@ -0,0 +1 @@
+long c_marker(void) { return 42; }

examples/1662-platform-build-pipeline-queries.sx

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+#import "modules/std.sx";
+#import "modules/build.sx";
+#import "modules/std/build.sx";
+
+// P5.2 smoke test — the build-pipeline metadata queries (`c_object_paths` /
+// `link_libraries`) run on the comptime VM and return a `List(string)` the VM
+// builds in flat memory. Registered as a post-link callback (which runs on the
+// VM — core.invokeByFuncId), so the lists are live there.
+//
+// The `#import c` source compiles to one `.o`, so `c_object_paths()` must return
+// a one-element list whose single string is the (non-empty) object path. AOT
+// snapshots the executed binary, so success is observed via the build exit code:
+// the callback returns true only if the VM-built list is well-formed.
+
+#import c {
+    #source "1662-platform-build-pipeline-queries.c";
+};
+
+c_marker :: () -> i64 extern;
+
+check :: () -> bool abi(.compiler) {
+    objs := c_object_paths();
+    libs := link_libraries();
+    if objs.len != 1 { return false; }
+    if objs.items[0].len == 0 { return false; }
+    // `link_libraries` must be a well-formed (possibly empty) list — touch each
+    // entry so a malformed fat pointer would fault rather than pass silently.
+    sum : i64 = 0;
+    i : i64 = 0;
+    while i < libs.len { sum += libs.items[i].len; i += 1; }
+    return sum >= 0;
+}
+
+configure :: () abi(.compiler) {
+    opts := build_options();
+    opts.set_post_link_callback(check);
+}
+#run configure();
+
+main :: () { print("runtime main: {}\n", c_marker()); }

examples/expected/1662-platform-build-pipeline-queries.build

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+{ "aot": true }

examples/expected/1662-platform-build-pipeline-queries.exit

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+0

examples/expected/1662-platform-build-pipeline-queries.stderr

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+

examples/expected/1662-platform-build-pipeline-queries.stdout

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+runtime main: 42