explore: shard tracks into per-year files for progressive loading

bake_tracks now writes tracks_YYYY.json shards + tracks_index.json manifest
instead of a single monolithic tracks.json. API /api/me/tracks returns the
manifest; /api/me/tracks/{year} serves individual shards. Explore.svelte
fetches the two most recent years eagerly then streams the rest in the
background so the map renders immediately with recent data.

bincio/explore.py

@@ -1,7 +1,8 @@
 """Pre-bake per-handle GPS tracks for the Explore page.
 
 Reads all activity GeoJSON files for a handle, applies RDP simplification,
-and writes a single tracks.json for fast client-side heatmap rendering.
+and writes per-year tracks_YYYY.json shards plus a tracks_index.json manifest
+for progressive client-side loading.
 """
 from __future__ import annotations
 
@@ -11,7 +12,7 @@ from pathlib import Path
 
 from bincio.extract.simplify import _rdp_mask
 
-_VERSION     = 1
+_VERSION     = 2
 _RDP_EPSILON = 0.0001   # ~10 m on the ground
 
 
@@ -88,14 +89,54 @@ def bake_tracks(handle: str, data_dir: Path) -> int:
 
     tracks.sort(key=lambda t: t["date"], reverse=True)
 
-    out = data_dir / handle / "tracks.json"
-    out.write_text(
+    user_dir = data_dir / handle
+    now = int(time.time())
+
+    # Group into per-year buckets
+    by_year: dict[str, list] = {}
+    for t in tracks:
+        year = t["date"][:4] or "0000"
+        by_year.setdefault(year, []).append(t)
+
+    # Remove stale year shards that no longer have data
+    for old in user_dir.glob("tracks_*.json"):
+        stem = old.stem  # e.g. "tracks_2024" or "tracks_index"
+        if stem == "tracks_index":
+            continue
+        year_part = stem[len("tracks_"):]
+        if year_part not in by_year:
+            old.unlink(missing_ok=True)
+
+    # Write per-year shards
+    for year, year_tracks in by_year.items():
+        shard_path = user_dir / f"tracks_{year}.json"
+        shard_path.write_text(
+            json.dumps({
+                "v":            _VERSION,
+                "handle":       handle,
+                "year":         year,
+                "generated_at": now,
+                "tracks":       year_tracks,
+            }),
+            encoding="utf-8",
+        )
+
+    # Write manifest
+    years_sorted = sorted(by_year.keys(), reverse=True)
+    index_path = user_dir / "tracks_index.json"
+    index_path.write_text(
         json.dumps({
             "v":            _VERSION,
             "handle":       handle,
-            "generated_at": int(time.time()),
-            "tracks":       tracks,
+            "generated_at": now,
+            "total":        len(tracks),
+            "years":        years_sorted,
         }),
         encoding="utf-8",
     )
+
+    # Remove legacy monolithic file if present
+    legacy = user_dir / "tracks.json"
+    legacy.unlink(missing_ok=True)
+
     return len(tracks)