seo: full audit fixes — meta, og, schema, canonical, sitemap, headings, alts

- Per-page title/description/canonical/OG tags: about, checkout (per-plan),
  terms, enterprise-terms, success all get unique SEO blocks
- Homepage title updated to em-dash form; meta description adds CTA
- og:site_name added to all pages
- noindex/nofollow on checkout, success, account pages
- Sitemap (/sitemap.xml) with all public pages; robots.txt updated with
  Sitemap directive and Disallow for private paths
- Schema: WebSite type added, Organization gains logo ImageObject, SoftwareApplication
  gains url field, billingIncrement corrected to billingPeriod (ISO 8601 P1M),
  sameAs gains x.com/neurontechai alongside GitHub
- marked.min.js given defer attribute (was render-blocking)
- page_head refactored into page_head_base + page_seo_block + page_open_seo
  for clean inner-page overrides without duplicating the CSS/script block

dist/page_schema.c

@@ -10,9 +10,18 @@ el_val_t page_schema(void) {
     "    \"@context\": \"https://schema.org\",\n"
     "    \"@graph\": [\n"
     "      {\n"
+    "        \"@type\": \"WebSite\",\n"
+    "        \"name\": \"Neuron\",\n"
+    "        \"url\": \"https://neurontechnologies.ai\"\n"
+    "      },\n"
+    "      {\n"
     "        \"@type\": \"Organization\",\n"
     "        \"name\": \"Neuron, LLC\",\n"
     "        \"url\": \"https://neurontechnologies.ai\",\n"
+    "        \"logo\": {\n"
+    "          \"@type\": \"ImageObject\",\n"
+    "          \"url\": \"https://neurontechnologies.ai/assets/brand/neuron-wordmark-on-light@2x.png\"\n"
+    "        },\n"
     "        \"founder\": {\n"
     "          \"@type\": \"Person\",\n"
     "          \"name\": \"Will Anderson\",\n"
@@ -20,11 +29,15 @@ el_val_t page_schema(void) {
     "        },\n"
     "        \"description\": \"Neuron builds AI that runs on your machine, builds a memory over time, and gets sharper the longer you use it. One builder. Built different.\",\n"
     "        \"foundingDate\": \"2026\",\n"
-    "        \"sameAs\": [\"https://github.com/neuron-technologies\"]\n"
+    "        \"sameAs\": [\n"
+    "          \"https://github.com/neuron-technologies\",\n"
+    "          \"https://x.com/neurontechai\"\n"
+    "        ]\n"
     "      },\n"
     "      {\n"
     "        \"@type\": \"SoftwareApplication\",\n"
     "        \"name\": \"Neuron\",\n"
+    "        \"url\": \"https://neurontechnologies.ai\",\n"
     "        \"applicationCategory\": \"AIApplication\",\n"
     "        \"operatingSystem\": \"macOS, Windows, Linux\",\n"
     "        \"offers\": [\n"
@@ -39,7 +52,7 @@ el_val_t page_schema(void) {
     "            \"name\": \"Professional\",\n"
     "            \"price\": \"19\",\n"
     "            \"priceCurrency\": \"USD\",\n"
-    "            \"billingIncrement\": \"monthly\"\n"
+    "            \"billingPeriod\": \"P1M\"\n"
     "          },\n"
     "          {\n"
     "            \"@type\": \"Offer\",\n"
@@ -67,7 +80,7 @@ el_val_t page_schema(void) {
     "            \"name\": \"What is Neuron?\",\n"
     "            \"acceptedAnswer\": {\n"
     "              \"@type\": \"Answer\",\n"
-    "              \"text\": \"Neuron is an AI that runs on your machine and builds a persistent memory over time. Every other AI forgets you when you close the tab. Neuron doesn't. The longer you use it, the less you have to explain.\"\n"
+    "              \"text\": \"Neuron is an AI that runs on your machine and builds a persistent memory over time. Every other AI forgets you when you close the tab. Neuron doesn't. The longer you use it, the less you have to explain.\"\n"
     "            }\n"
     "          },\n"
     "          {\n"