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.

src/ir/comptime_vm.zig

@@ -1480,6 +1480,27 @@ pub const Vm = struct {
                 return self.failMsg("comptime emit_object: no object was emitted (object_path unset)");
             return try self.makeStringValue(table, path);
         }
+        // `link(objects, output, libraries, frameworks, flags, target)` — the one
+        // genuine ACTION: dispatch to the host-installed linker (the VM can't link
+        // itself). Void return (the build callback isn't fallible — Phase 5
+        // decision); a link failure bails loudly → hard build error. `ref_types`
+        // gives each List(string) arg its concrete type for the flat-memory reader.
+        if (std.mem.eql(u8, name, "link")) {
+            if (args.len != 6) return self.failMsg("comptime link: expected (objects, output, libraries, frameworks, flags, target)");
+            const bc = self.build_config orelse
+                return self.failMsg("comptime link: no build config threaded into the VM");
+            const hooks = bc.build_hooks orelse
+                return self.failMsg("comptime link: no build hooks installed (link is a post-codegen-only action)");
+            const objects = try self.readStringList(table, ref_types[args[0].index()], frame.get(args[0].index()));
+            const output = try self.readStringArg(table, frame.get(args[1].index()));
+            const libraries = try self.readStringList(table, ref_types[args[2].index()], frame.get(args[2].index()));
+            const frameworks = try self.readStringList(table, ref_types[args[3].index()], frame.get(args[3].index()));
+            const flags = try self.readStringList(table, ref_types[args[4].index()], frame.get(args[4].index()));
+            const target_str = try self.readStringArg(table, frame.get(args[5].index()));
+            hooks.link(hooks.ctx, objects, output, libraries, frameworks, flags, target_str) catch
+                return self.failMsg("comptime link: linking failed");
+            return @as(Reg, null_addr); // void
+        }
         return null; // not a known compiler function → caller bails to legacy
     }
 
@@ -2258,4 +2279,31 @@ pub const Vm = struct {
         try self.writeField(table, addr + fieldOffset(table, list_ty, 2), cap_fty, n);
         return addr;
     }
+
+    /// Read a `string` argument (a `{ptr, len}` fat pointer at `val`) as a host
+    /// `[]const u8`. The bytes are a VIEW into flat memory (Addr is a real host
+    /// pointer over a stable arena), valid for the duration of the call.
+    fn readStringArg(self: *Vm, table: *const types.TypeTable, val: Reg) Error![]const u8 {
+        const len: usize = @intCast(try self.sliceLen(val));
+        if (len == 0) return "";
+        return try self.machine.bytes(try self.sliceData(table, val), len);
+    }
+
+    /// Read a `List(string)` aggregate (at `addr`) into a host `[][]const u8` —
+    /// the inverse of `makeStringList`. Element string bytes are VIEWS into flat
+    /// memory (stable arena); the outer array is gpa-allocated (freed at
+    /// `Vm.deinit`). Used by the `link` primitive to read its List args.
+    fn readStringList(self: *Vm, table: *const types.TypeTable, list_ty: TypeId, addr: Addr) Error![]const []const u8 {
+        if (list_ty.isBuiltin() or table.get(list_ty) != .@"struct")
+            return self.failMsg("comptime List reader: arg type is not a List struct");
+        const items_ptr = try self.machine.readWord(addr + fieldOffset(table, list_ty, 0), table.pointer_size);
+        const len: usize = @intCast(try self.machine.readWord(addr + fieldOffset(table, list_ty, 1), 8));
+        const str_size = table.typeSizeBytes(.string);
+        const out = self.gpa.alloc([]const u8, len) catch return self.failMsg("comptime List reader: out of memory");
+        var i: usize = 0;
+        while (i < len) : (i += 1) {
+            out[i] = try self.readStringArg(table, items_ptr + i * str_size);
+        }
+        return out;
+    }
 };