feat(native-el-ui): rewrite checkout/main to use el-html vessel, fix page_close linker error

- checkout.el and main.el: replace raw import of el-html vessel with direct
  extern fn declarations; implementations come from dist/elhtml_impl.c (c_source)
- Add src/elhtml.el as reference file (all el-html extern fn declarations; not imported)
- dist/elhtml_impl.c: pre-compiled el-html vessel C (strips int main + sample global)
- dist/page_close.c: pre-compiled page_close implementation; elc OOMs after emitting
  the ~71KB page_open body and silently drops page_close, so supply it as c_source
- manifest.el: add elhtml_impl.c and page_close.c as c_source entries
- .gitignore: un-ignore dist/elhtml_impl.c and dist/page_close.c

manifest.el

@@ -11,4 +11,6 @@ build {
     output "dist/"
     c_source "dist/web_stubs.c"
     c_source "dist/vessel_stubs.c"
+    c_source "dist/elhtml_impl.c"
+    c_source "dist/page_close.c"
 }