feat(site): BitGate landing page, integration guide, and live demo

A sibling of the BitLogin design system rather than a twin: the same
ink/paper/brass foundation and serif display, with verdigris — the patina brass
takes as it ages — where BitLogin uses its violet seal.

- index: the signature visual is the product's whole argument, one listing
  reaching four surfaces and getting four different verdicts
- docs: five-step integration, elements table, presets, common mistakes
- demo: the real engine running in the browser against an in-memory transport,
  so nothing is fetched or published. Add reports, restrict the seller, switch
  moderator identities, and watch every surface re-decide.

The widget bundle now re-exports the runtime and core entry points, because a
static page has no package manager — anything it might need has to be reachable
from the one file it loads.

Also fixes the site build reusing a stale widget bundle. It only rebuilt when
the file was missing, which is precisely how a broken demo ships: the page
loads, the module resolves, and an export added since the last build is
silently absent. Caught by running it.

vercel.json mirrors BitLogin's security headers, with connect-src widened to
wss: for relay traffic.

package-lock.json

@@ -50,6 +50,10 @@
       "resolved": "packages/runtime",
       "link": true
     },
+    "node_modules/@bitgate/site": {
+      "resolved": "packages/site",
+      "link": true
+    },
     "node_modules/@bitgate/testing": {
       "resolved": "packages/testing",
       "link": true
@@ -1798,6 +1802,10 @@
         "@bitgate/nostr": "*"
       }
     },
+    "packages/site": {
+      "name": "@bitgate/site",
+      "version": "0.1.0"
+    },
     "packages/testing": {
       "name": "@bitgate/testing",
       "version": "0.1.0",