[patch] Keep both values when two keys standardize to the same name

[matt-edmondson]

Fixes #136

Problem

NameStandardizer.StandardizePropertyNames wrote each renamed property with standardizedFrontmatter[name] = value. When two keys resolved to the same standard name, the later one overwrote the earlier one without any warning. With the default options, a document carrying both author and creator lost its author.

Change

All in Frontmatter/NameStandardizer.cs:

• Name lookup extracted: working out a key's standard name (standard-name lower-casing, cache, known mappings, normalized match) now lives in GetStandardName. The rules are the same as before.
• Rename only when free: a key is renamed only when its standard name is free, meaning it hasn't already been written and isn't the exact name of a key still to come. Otherwise the key keeps its original name. The lower-casing branch (Title → title) goes through the same guard.

Tests

• Added StandardNamingCollisionTests. It covers the three rows in the issue, plus the alias-before-standard-name order and a check that a lone alias is still renamed.
• With NameStandardizer.cs reverted, the four collision tests fail. The lone-alias test passes either way, as expected.
• With the fix, all 189 tests pass on net10.0.

Notes

• This PR is independent of [patch] Stop the merge-strategy cache letting one call decide a key for every later call #155 (Merge-strategy cache is keyed only on the property name, so an Aggressive run makes a later Conservative run drop properties #130). The two change different files, and both branch from main.
• Aggressive/Maximum merging swaps the values of two keys that normalize alike (e.g. related / page_related) instead of merging them #131 (Aggressive merging swapping values) is a separate fix in PropertyMerger and is not touched here.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KrGyxYxnoFgAkP1ENResCJ

---

Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud