fix: clear all latent deprecation + resource warnings

Three real issues surfaced when running the suite with strict warnings:

1. src/core/format_standardize.py: ``datetime.utcfromtimestamp`` is
   deprecated in CPython 3.12 and slated for removal. Replace with
   ``datetime.fromtimestamp(ts, tz=timezone.utc)``. Output for the
   date-only format codes we use is byte-identical.

2. src/core/io.py: ``list_sheets`` leaked the openpyxl file handle by
   returning ``xl.sheet_names`` from an unclosed ``pd.ExcelFile``.
   Wrap in a ``with`` block so the FD closes deterministically — also
   prevents the Windows-only "file is locked" repro path.

3. tests/test_corpus.py: ``TestXlsxPollution.workbook`` fixture
   returned the bare ``pd.ExcelFile`` instead of yielding + closing.
   Convert to a yield-and-finally pattern so the class-scoped handle
   isn't leaked across the whole test file.

Also harden pytest.ini's warning policy: escalate
``ResourceWarning`` from ``src`` to an error, alongside the existing
``DeprecationWarning`` rule. Third-party warnings stay filtered — we
can't fix pandas/openpyxl/streamlit churn from here.

All 1916 tests pass under the strict filter; full and split runs
(``pytest``, ``pytest -m 'not gui'``, ``pytest -m gui``) all clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pytest.ini

@@ -14,8 +14,12 @@ markers =
     fixture_sweep: parametrized sweep over the test-cases/ folder
     gui: Streamlit AppTest-driven tests (live in tests/gui/)
 
-# Warnings discipline: fail on unexpected DeprecationWarning from our own
-# code, but tolerate third-party deprecations that we can't fix.
+# Warnings discipline: fail on any DeprecationWarning *or* ResourceWarning
+# from our own ``src`` package so a leaked file handle or stale stdlib call
+# can't slip in unnoticed. Tolerate third-party deprecations / resource
+# warnings — we can't fix pandas / openpyxl / streamlit churn from here.
 filterwarnings =
     error::DeprecationWarning:src
+    error::ResourceWarning:src
     ignore::DeprecationWarning
+    ignore::ResourceWarning