feat: parse and validate FIT containers

tests/fit/test_rewriter_validation.py

@@ -0,0 +1,21 @@
+from pathlib import Path
+
+import pytest
+
+from app.fit.rewriter import FitFormatError, is_fit_file
+from tests.fit.builders import make_fit
+
+
+def test_valid_12_and_14_byte_headers(tmp_path: Path) -> None:
+    for size in (12, 14):
+        path = tmp_path / f"valid-{size}.fit"
+        path.write_bytes(make_fit(b"", header_size=size))
+        assert is_fit_file(path) is True
+
+
+def test_bad_file_crc_is_rejected(tmp_path: Path) -> None:
+    payload = bytearray(make_fit(b""))
+    payload[-1] ^= 0xFF
+    path = tmp_path / "bad.fit"
+    path.write_bytes(payload)
+    assert is_fit_file(path) is False