perf: year-shard index.json to cut initial load from MBs to ~1 year

merge_all/_merged/index.json is now a shard manifest; activities are
split into index-{year}.json files. The feed loads only the most-recent
year on first paint (~200 activities instead of all of them). Older
years are fetched lazily when the user clicks "Load older activities".

Also strips best_efforts / best_climb_m / source from shard files —
these fields are aggregation inputs only, never read by the feed UI.

tests/test_merge.py

@@ -9,6 +9,18 @@ import pytest
 from bincio.render.merge import apply_sidecar, merge_all, merge_one, parse_sidecar
 
 
+def _load_merged_activities(merged_dir: Path) -> dict:
+    """Load all activities from year-sharded merged index. Returns id→dict map."""
+    root = json.loads((merged_dir / "index.json").read_text())
+    all_acts = list(root.get("activities", []))
+    for shard in root.get("shards", []):
+        shard_path = merged_dir / shard["url"]
+        if shard_path.exists():
+            sub = json.loads(shard_path.read_text())
+            all_acts.extend(sub.get("activities", []))
+    return {a["id"]: a for a in all_acts}
+
+
 # ── parse_sidecar ─────────────────────────────────────────────────────────────
 
 
@@ -176,8 +188,7 @@ def test_merge_all_private_filtered_from_index(data_dir):
     (edits / "2024-01-01T080000Z-morning-ride.md").write_text("---\nprivate: true\n---\n")
     merge_all(data_dir)
 
-    index = json.loads((data_dir / "_merged" / "index.json").read_text())
-    activities = {a["id"]: a for a in index["activities"]}
+    activities = _load_merged_activities(data_dir / "_merged")
     # unlisted activities are kept in the index; filtering is client-side
     assert "2024-01-01T080000Z-morning-ride" in activities
     assert activities["2024-01-01T080000Z-morning-ride"]["privacy"] == "unlisted"
@@ -191,8 +202,11 @@ def test_merge_all_highlight_sorts_first(data_dir):
     (edits / "2024-01-01T080000Z-morning-ride.md").write_text("---\nhighlight: true\n---\n")
     merge_all(data_dir)
 
-    index = json.loads((data_dir / "_merged" / "index.json").read_text())
-    ids = [a["id"] for a in index["activities"]]
+    # Highlighted activity must be first within its year shard
+    merged_dir = data_dir / "_merged"
+    root = json.loads((merged_dir / "index.json").read_text())
+    shard_path = merged_dir / root["shards"][0]["url"]
+    ids = [a["id"] for a in json.loads(shard_path.read_text())["activities"]]
     assert ids[0] == "2024-01-01T080000Z-morning-ride"