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_pipeline.py

@@ -83,10 +83,15 @@ class TestPipeline:
         merge_all(data_root / "brut")
 
         for handle in ("dave", "brut"):
-            merged = json.loads((data_root / handle / "_merged" / "index.json").read_text())
-            assert len(merged["activities"]) >= 8, (
-                f"Expected ≥8 merged activities for {handle}"
-            )
+            merged_dir = data_root / handle / "_merged"
+            root = json.loads((merged_dir / "index.json").read_text())
+            # Root index now has year shards; collect all activities across them
+            all_acts: list = list(root.get("activities", []))
+            for shard in root.get("shards", []):
+                sp = merged_dir / shard["url"]
+                if sp.exists():
+                    all_acts.extend(json.loads(sp.read_text()).get("activities", []))
+            assert len(all_acts) >= 8, f"Expected ≥8 merged activities for {handle}"
 
     def test_root_manifest(self, data_root):
         from bincio.render.cli import _user_dirs, _write_root_manifest