Print Mayer bond orders in Gaussian jobs

[kfir4444]

Stack 2/5 — based on #951, review that one first.

What

IOp(2/9=2000) → IOp(2/9=2000,6/80=1) in the Gaussian route section.

Why

6/80=1 makes Gaussian print the Mayer atomic bond orders that #951 parses. It is a post-SCF population analysis and its cost is negligible. Orca prints the same analysis by default, so there is no equivalent change on that side.

Making it unconditional rather than opt-in means every Gaussian log ARC produces from here on can be used for connectivity perception, including the IRC endpoint optimizations in the later PRs of this stack.

Note for reviewers

This changes every Gaussian input ARC writes, which is why 25 expected route strings in gaussian_test.py change with it. The expected values changed because the generator deliberately changed — no assertion was relaxed to accommodate new behaviour, and the change is a single token in one template line.

arc/job/adapters/psi_4.py carries a copied Gaussian-style route line containing the same IOp(2/9=2000). It was deliberately left alone as out of scope.

Testing

pytest arc/job/ — 985 passed, 3 failed. All three failures (test_which, two xTB tests) are environment-dependent and reproduce on main.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@                     Coverage Diff                     @@
##             mayer_bond_orders_parser     #952   +/-   ##
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  Coverage                            ?   63.62%           
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  Lines                               ?    38443           
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  Hits                                ?    24461           
  Misses                              ?    11061           
  Partials                            ?     2921           

Flag	Coverage Δ
functionaltests	63.62% <ø> (?)
unittests	63.62% <ø> (?)

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