(feat) Discord build share

docs/architecture.md

@@ -187,6 +187,40 @@ boundary. An owned Build page links to `/guides/new?build=<id>`, where the
 server verifies ownership and initializes a removable Build block in a new
 private Guide.
 
+### Discord-compatible Build sharing experiment
+
+Persisted public and unlisted Builds expose absolute Open Graph metadata for
+their canonical `/builds/[slug]` URL. Their `og:image` and Twitter card metadata
+reference the same-origin 1200 × 630 PNG at `/builds/[slug]/preview.png` so
+Discord and other link unfurlers can render a Build summary. Private Builds set
+`noindex,nofollow`, expose no crawler image, and provide no usable sharing
+control.
+
+The preview endpoint derives its image from canonical Build facts and the
+current known-stat summary in the shared Build projection. It includes the Build
+title, author, visibility, direct URL, equipped weapon names, core totals, and
+up to three supporting Attributes. The renderer performs no external fetches. It
+normalizes and bounds every text field by grapheme count, escapes text before
+inserting it into XML, builds a text-only SVG, and rasterizes that SVG to PNG
+with Sharp. Production installs DejaVu Sans from the Debian package and the
+renderer names that fixed family explicitly.
+
+The endpoint hashes a versioned serialization of the image input for its bounded
+in-process render cache and derives the HTTP ETag from the resulting PNG bytes.
+A bounded request queue limits concurrent database projections and Sharp work.
+Responses require cache revalidation so a visibility change to Private blocks
+the next request instead of leaving a stale response window. Matching
+conditional requests receive `304 Not Modified`. Private, missing, and
+inaccessible Builds receive no preview. Unlisted images send the
+`X-Robots-Tag: noindex` response header.
+
+For eligible Builds, the Build Uplink invokes the native Web Share API as the
+primary action when the browser supports it. An explicit Copy Link action
+remains available for every public or unlisted persisted Build. This experiment
+does not install a Discord bot, post to Discord, extend Discord OAuth scopes, or
+store Discord tokens or webhooks. Discord receives only a link that an Agent
+chooses to share and may fetch the public metadata and same-origin preview.
+
 ## Public data corrections and discussion
 
 Every public database card links to its canonical ISAC record with an exact