perf: combined feed index for multi-user global feed

Instead of the browser resolving 20+ user shards recursively (~27 MB),
generate a pre-sorted feed.json at merge time with 50 activities per
page. The global feed loads one ~30 KB file on first paint; "Load more"
fetches subsequent pages (feed-2.json, feed-3.json, etc.).

Per-user profile pages still use year-sharded loadIndexPaged as before.

bincio/render/cli.py

@@ -133,6 +133,11 @@ def _write_root_manifest(data: Path) -> None:
     root.write_text(json.dumps(manifest, indent=2))
     console.print(f"Root manifest updated: [cyan]{len(users)}[/cyan] user shard(s)")
 
+    if len(users) > 1:
+        from bincio.render.merge import write_combined_feed
+        n = write_combined_feed(data)
+        console.print(f"Combined feed: [cyan]{n}[/cyan] activities across all users")
+
 
 def _link_data(site: Path, data: Path) -> None:
     """Symlink site/public/data → data root (each user has their own _merged/)."""

bincio/render/merge.py

@@ -320,3 +320,85 @@ def _write_year_shards(merged_dir: Path, activities: list[dict], index_meta: dic
         "activities": [],
     }
     (merged_dir / "index.json").write_text(json.dumps(root_doc, indent=2, ensure_ascii=False))
+
+
+FEED_PAGE_SIZE = 50
+
+# Extra fields stripped from the combined feed — preview_coords is the biggest
+# contributor (~24% of shard size) but the feed cards need it for thumbnails,
+# so we keep it.  mmp is never displayed in feed cards.
+_COMBINED_FEED_STRIP = _FEED_STRIP | {"mmp"}
+
+
+def write_combined_feed(data_dir: Path) -> int:
+    """Build data_dir/feed.json — the N most recent activities across all users.
+
+    The global feed page loads this single file instead of resolving 20+ user
+    shards recursively.  Returns the number of activities written.
+    """
+    user_dirs = sorted(
+        p for p in data_dir.iterdir()
+        if p.is_dir() and (p / "activities").exists()
+    )
+
+    all_activities: list[dict] = []
+    for user_dir in user_dirs:
+        handle = user_dir.name
+        merged = user_dir / "_merged"
+        index_path = merged / "index.json" if merged.exists() else user_dir / "index.json"
+        if not index_path.exists():
+            continue
+
+        index = json.loads(index_path.read_text(encoding="utf-8"))
+        shards = index.get("shards", [])
+        activities = index.get("activities", [])
+
+        if shards:
+            year_shards = sorted(
+                [s for s in shards if re.match(r"index-\d{4}\.json$", s.get("url", ""))],
+                key=lambda s: s["url"],
+                reverse=True,
+            )
+            base = index_path.parent
+            for shard in year_shards[:2]:
+                shard_path = base / shard["url"]
+                if shard_path.exists():
+                    shard_data = json.loads(shard_path.read_text(encoding="utf-8"))
+                    for a in shard_data.get("activities", []):
+                        a_tagged = {**a, "handle": handle}
+                        detail_url = a_tagged.get("detail_url", "")
+                        if detail_url and not detail_url.startswith("http") and not detail_url.startswith("/"):
+                            merged_rel = f"{handle}/_merged/" if merged.exists() else f"{handle}/"
+                            a_tagged["detail_url"] = merged_rel + detail_url
+                        track_url = a_tagged.get("track_url", "")
+                        if track_url and not track_url.startswith("http") and not track_url.startswith("/"):
+                            merged_rel = f"{handle}/_merged/" if merged.exists() else f"{handle}/"
+                            a_tagged["track_url"] = merged_rel + track_url
+                        all_activities.append(a_tagged)
+        else:
+            for a in activities:
+                all_activities.append({**a, "handle": handle})
+
+    all_activities.sort(key=lambda a: a.get("started_at", ""), reverse=True)
+
+    # Remove stale feed pages
+    for f in data_dir.glob("feed*.json"):
+        f.unlink()
+
+    if not all_activities:
+        return 0
+
+    pages = [all_activities[i:i + FEED_PAGE_SIZE] for i in range(0, len(all_activities), FEED_PAGE_SIZE)]
+    for page_num, page in enumerate(pages):
+        slim = [{k: v for k, v in a.items() if k not in _COMBINED_FEED_STRIP} for a in page]
+        fname = "feed.json" if page_num == 0 else f"feed-{page_num + 1}.json"
+        doc = {
+            "bas_version": "1.0",
+            "page": page_num + 1,
+            "total_pages": len(pages),
+            "total_activities": len(all_activities),
+            "activities": slim,
+        }
+        (data_dir / fname).write_text(json.dumps(doc, indent=2, ensure_ascii=False))
+
+    return len(all_activities)