wip(E4): partial source-pin + non-transitive flip [stdlib E4 attempt-1 WIP checkpoint]

Incomplete WIP from a worker killed at the 55-min wall (large blast radius:
core source-pin + ~8 example migrations + ~10 library module migrations).
Committed so the resumed session continues on a clean tree. May not build.

library/modules/std/json.sx

@@ -58,6 +58,9 @@
 // =====================================================================
 
 #import "modules/std.sx";
+// `Array`/`Object` methods take an explicit `alloc: Allocator`; bare-import
+// visibility is non-transitive, so the module that names the type imports it.
+#import "modules/allocators.sx";
 #import "modules/fs.sx";
 
 // The writer's failure contract: a too-small caller buffer (Overflow) or

library/modules/std/objc_block.sx

@@ -22,6 +22,12 @@
 // caller returns. If you need that, ship a `Block_copy`-backed sibling
 // API and use it instead.
 
+// `build_block_convert` (below) is a comptime metaprogram that emits sx source
+// with `concat` / `int_to_string`; those live in std.sx. A metaprogram body
+// resolves its bare names in its OWN module (issue 0106), so this module must
+// import std.sx itself rather than relying on the call site's visibility.
+#import "modules/std.sx";
+
 // Standard 32-byte block header plus two trailing slots for the sx closure
 // it wraps. Total = 48 bytes.
 Block :: struct {