fix(build): c_source stubs, manifest directives, gallery module-level global

Add window/neuronCheckoutFree stubs to web_stubs.c — needed to link
without undefined symbol errors on macOS (vessel stubs were already
handled, web platform stubs were not).

Add c_source directives to manifest.el so elb includes web_stubs.c and
vessel_stubs.c in the link — requires the matching elb update in
foundation/el PR #46.

Move gallery_share_allowlist from module scope into gallery_page() to
prevent elc from emitting a second main() for the gallery module, which
caused a duplicate-symbol link error when combined with main.el.

Update elc-linux-amd64 binary (rebuilt with RBrace fix from PR #46).

dist/web_stubs.c

@@ -325,3 +325,9 @@ el_val_t gcs_read(el_val_t bucket, el_val_t object_name) {
     if (!resp) return EL_STR("");
     return resp;
 }
+
+/* Browser JS interop stubs — server-side no-ops for checkout.el browser globals.
+ * checkout.el contains JS inline: window.neuronCheckoutFree&&window.neuronCheckoutFree()
+ * which the El HTML parser exposes as C identifiers in the generated checkout.c. */
+el_val_t window = 0;
+el_val_t neuronCheckoutFree(el_val_t v) { (void)v; return 0; }