chore(ffi-linkage): post-stream polish — vscode keywords + vestigial param + extension metadata

Two post-stream follow-ups flagged in CHECKPOINT-EXTERN-EXPORT.md, plus a
reproducible vscode-extension packaging setup:

- parser: drop the vestigial `RuntimeClassPrefix.is_extern` field and
  `parseRuntimeClassDecl`'s `is_extern` param. Always false since the
  `#foreign` token was deleted; the postfix `extern`/`export` keyword is the
  sole reference-vs-define decider. No behavior change (644 corpus / 442 unit).
- vscode grammar: highlight `extern`/`export` as `storage.modifier.sx`.
- vscode packaging: declare `@vscode/vsce` as a devDep + add `package` /
  `vscode:prepublish` scripts so the vsix rebuilds reproducibly (was an
  ambient tool). Add repository/homepage/bugs (Gitea), icon (swipelab logo,
  256x256), galleryBanner, README with cover banner. Rebuilt the vsix.

editors/vscode/syntaxes/sx.tmLanguage.json

@@ -193,6 +193,10 @@
           "name": "keyword.other.sx",
           "match": "\\b(enum|struct)\\b"
         },
+        {
+          "name": "storage.modifier.sx",
+          "match": "\\b(extern|export)\\b"
+        },
         {
           "name": "keyword.operator.cast.sx",
           "match": "\\bxx\\b"