[scks1]

Summary

Two defects in DOCX math conversion, both reproduced on 0.1.9:

1. One bad equation aborted all math in the file. _pre_process_math
   replaced equations in a loop with no per-equation guard, so a single
   failing equation discarded every other equation's conversion in the
   same document part.

2. Unknown limit functions raised NotImplementedError.
   do_limlow raised for any limit function name not in its hardcoded
   table, even though do_fname already falls back to
   \operatorname{...} for unknown function names.

Changes

• pre_process.py: _replace_equations guards each equation
  replacement; a failing equation keeps its original OMML tag (inline and
  inside oMathPara), and the _pre_process_math loops guard each call
  so one tag cannot abort the remaining equations.
• omml.py: do_limlow stops raising NotImplementedError for an
  unknown limit function and falls back to
  \operatorname{<name>}_{<lim>}, following the \operatorname
  fallback do_fname already uses for unknown function names.

Scope note

This PR originally also fixed KeyError raised by five OMML handlers
(do_acc, do_bar, do_d, do_f, do_groupchr) when a properties
element (accPr, barPr, dPr, fPr, groupChrPr) is absent. Those
fixes and their tests have been withdrawn: #2445 already covers exactly
that defect, with tests of equal coverage. What remains here — the
per-equation isolation and the do_limlow fallback — is not covered by
#2445, and the isolation is also distinct from #2714, which drops a
failing block equation where this PR preserves its original OMML tag.

Tests

Three new tests in tests/test_docx.py, each failing on 0.1.9 and
passing with this change: one for the limLow fallback and two for
per-equation isolation. The isolation tests use a func element with
no fName, which still raises after these fixes, so they exercise the
guard itself.

Full run of tests/test_docx.py on the narrowed tree: 104 passed.

Disclosure: this contribution was prepared by Árbol, an AI assistant working under the direction of the account holder (scks1), who set the acceptance criteria and verified the results before submission.