Add neutron to PDGCode enum-to-string mapping

[Copilot]

The PDGCode enum names proton/anti_proton but only has n0/anti_n0 for the neutron — inconsistent and less readable.

• Add neutron = 2112 and anti_neutron = -2112 enum aliases in PDGCode and CompressedPDGCode
• Change string representations from "n0" → "neutron" and "anti_n0" → "anti_neutron"
• Existing n0/anti_n0 enum names kept as aliases — fully backward compatible

// Both work:
PDGCode::n0       // == 2112, still valid
PDGCode::neutron  // == 2112, new alias

Original prompt

---

This section details on the original issue you should resolve

<issue_title>no neutron in PDGCode enum-to-string</issue_title>
<issue_description>The PDGCode enum-to-string names a proton, but there is no neutron.

There are p0 and pi_plus. There is n0... should there be n_plus? We called n_plus a proton, it would make sense to say "neutron" instead of n0.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes no neutron in PDGCode enum-to-string #1218

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[rlcee]

@FNALbuild run build test

[FNALbuild]

Hi @Copilot,
You have proposed changes to files in these packages:

• DataProducts

which require these tests: build.

@Mu2e/write, @Mu2e/fnalbuild-users have access to CI actions on main.

📝 The author of this pull request is not a member of the Mu2e github organisation.

⌛ The following tests have been triggered for d0b0ace: build (Build queue - API unavailable)

About FNALbuild. Code review on Mu2e/Offline.

[FNALbuild]

☀️ The build tests passed at d0b0ace.

Test	Result	Details
test with	✅	Command did not list any other PRs to include
merge	✅	Merged d0b0ace at 627421c
build (prof)	✅	Log file. Build time: 04 min 06 sec
ceSimReco	✅	Log file.
g4test_03MT	✅	Log file.
transportOnly	✅	Log file.
POT	✅	Log file.
g4study	✅	Log file.
cosmicSimReco	✅	Log file.
cosmicOffSpill	✅	Log file.
ceSteps	✅	Log file.
ceDigi	✅	Log file.
muDauSteps	✅	Log file.
ceMix	✅	Log file.
rootOverlaps	✅	Log file.
g4surfaceCheck	✅	Log file.
trigger	❓	Log file. Return Code 1.
check_cmake	✅	Log file.
FIXME, TODO	✅	TODO (0) FIXME (0) in 4 files
clang-tidy	✅	0 errors 0 warnings
whitespace check	✅	no whitespace errors found

N.B. These results were obtained from a build of this Pull Request at d0b0ace after being merged into the base branch at 627421c.

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[rlcee]

In my reading of the code, this should work.