fix(reader_xlsx): add missing import subprocess (recalc silently disabled)

[Anai-Guo]

Problem

plugins/tools/_reader_xlsx.py calls subprocess.run(...) in _x_recalc_copy (lines 133 and 149), but the module never imports subprocess:

import datetime as _dt
import os
import re
import shutil
import tempfile
import xml.etree.ElementTree as _ET
import zipfile

Because the entire body of _x_recalc_copy is wrapped in try: ... except Exception: return None, the NameError: name 'subprocess' is not defined is swallowed silently rather than crashing.

Impact

_x_recalc_copy first checks if not shutil.which("soffice"): return None, so the subprocess.run calls are only reached when LibreOffice is installed. In exactly that case, the call raises NameError, is caught, and returns None. The caller then does:

new = _x_recalc_copy(path)
if new:
    ... # recalc done
else:
    mark_uncached = True
    head.append("`recalc: soffice unavailable; empty formula caches marked uncached`")

So on any machine where soffice is available, the formula-recalc feature silently reports "soffice unavailable" and never recalculates empty formula caches — the feature is effectively dead.

Fix

Add the missing import subprocess. One line, no behavior change beyond letting the recalc path run as intended.

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