ci: engines >=20 + .nvmrc; hard audit gate; boot check; bundle budget; eslint ratchet

- engines.node >=20.0.0 and .nvmrc mirroring .node-version (#54)
- npm audit --audit-level=high is a hard gate (tree is at 0) (#91)
- scripts/boot-check.mjs serves dist/ with vite preview and asserts /,
  config.json, the entry chunk and the Element Call bundle all load (#92)
- scripts/check-bundle-size.mjs enforces gzip budgets from
  scripts/bundle-budget.json (seeded +10%); fails PRs, warns on push (#96)
- check:eslint runs with --max-warnings 68 so the count can only go down;
  7 unused eslint-disable directives removed to get there (#97)

Fixes #54
Fixes #91
Fixes #92
Fixes #96
Fixes #97

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

package.json

@@ -5,14 +5,14 @@
   "main": "index.js",
   "type": "module",
   "engines": {
-    "node": ">=16.0.0"
+    "node": ">=20.0.0"
   },
   "scripts": {
     "start": "vite",
     "build": "vite build",
     "preview": "vite preview",
     "lint": "npm run check:eslint && npm run check:prettier",
-    "check:eslint": "eslint src/*",
+    "check:eslint": "eslint src/* --max-warnings 68",
     "check:prettier": "prettier --check .",
     "fix:prettier": "prettier --write .",
     "typecheck": "tsc --noEmit",