[patch] Stop the merge-strategy cache letting one call decide a key for every later call

[matt-edmondson]

Fixes #130

Problem

PropertyMerger.PropertyMergeCache was a process-wide cache keyed only on the property name. But a key's canonical name also depends on the strategy and, for Aggressive and Maximum, on the other keys in the same document. So the first call to reach a key decided how every later call treated it:

• After an Aggressive run, a Conservative run dropped custom_title.
• After a Conservative run, an Aggressive run stopped merging.
• A key merged in one document was renamed in a later document that had nothing to merge it with.

Change

• Cache removed: GetCanonicalName works out the name on every call. The work is a dictionary lookup plus a small normalization over a handful of keys, which is the lower-risk option the triage recommended.
• Test hack removed: PropertyMergerTests, SingleCharacterKeyTests and DecorationOnlyKeyTests no longer clear the cache by reflection. That clearing was what hid the bug.

Tests

• Added MergeStrategyIsolationTests, which runs strategies and documents back to back without clearing anything. It covers:
  • Aggressive then Conservative: the acceptance case from the issue, checked at both the merger level and the CombineFrontmatter level
  • Conservative then Aggressive
  • a key merged in one document, then appearing alone in a later one
• With PropertyMerger.cs reverted, 3 of the 4 new tests fail. The cross-document test passes either way.
• With the fix, all 188 tests pass on net10.0.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KrGyxYxnoFgAkP1ENResCJ

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

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