fix: validate activity file exists before treating cached hash as known

The dedup cache (.bincio_cache.json) persists source hashes across runs.
If the activities/ directory is wiped (--fresh in another session, or macOS
clearing /tmp after a restart) while the cache file survives at the user dir
level, re-running extract skips all files as "already extracted" and leaves
activities/ empty. Now only hashes whose corresponding .json file is present
on disk are treated as known, so missing files are always re-extracted.

bincio/extract/cli.py

@@ -169,7 +169,13 @@ def extract(
         console.print(f"Loaded Strava metadata from [cyan]{cfg.metadata_csv.name}[/cyan].")
 
     dedup = DedupIndex(output_dir=cfg.output_dir)
-    known_hashes: frozenset = frozenset(dedup._by_hash.keys())
+    # Only skip files whose output actually exists — the cache can outlive a
+    # --fresh wipe or manual deletion of the activities directory.
+    _acts_dir = cfg.output_dir / "activities"
+    known_hashes: frozenset = frozenset(
+        h for h, act_id in dedup._by_hash.items()
+        if (_acts_dir / f"{act_id}.json").exists()
+    )
 
     n_workers = workers or cfg.workers or os.cpu_count() or 4
     console.print(f"Using [bold]{n_workers}[/bold] worker processes.")