Optimize reactor sort, model pool, and phase comparator performance

[gnodet]

Summary

JFR profiling of a 4,383-module reactor build revealed three hotspots that together consume ~35% of CPU time. All three share the same root cause: O(n) linear scans used inside tight loops or comparators.

Fixes

• DefaultGraphBuilder (23.5% CPU): Three call sites use result.sort(comparing(sortedProjects::indexOf)) where ArrayList.indexOf() is O(n), causing O(n² log n) total MavenProject.equals() calls (~230M for 4,383 modules). Replaced with a HashMap<MavenProject, Integer> index built in O(n), reducing each sort comparison to O(1).

• DefaultModelObjectPool (~8% CPU, ~2.4 GB allocation pressure):

  • getPooledTypes() re-parses a comma-separated property string into a new Stream → Set on every process() call (hundreds of thousands of times). Cached at construction time.
  • PoolKey.dependencyHashCode() uses Objects.hash() with 12 arguments, allocating a new Object[12] on every call. Inlined the hash computation.
  • Added hashCode fast-rejection to PoolKey.equals() to skip expensive deep equality when hashes differ.

• PhaseComparator (~2% CPU): List.indexOf() in compare() is O(n) per call. Pre-built a HashMap<String, Integer> in the constructor for O(1) phase index lookups.

JFR Profile (before these fixes)

Rank	Method	CPU %	Root Cause
1	MavenProject.equals	23.5%	Called from ArrayList.indexOf in DefaultGraphBuilder sort
2	DefaultModelObjectPool$PoolKey.*	~8%	Stream re-parse + Objects.hash varargs
3	DefaultDependencyManagementImporter.importManagement	5.8%	(not addressed — model-level, not a data-structure issue)
4	PhaseComparator.compare	~2%	List.indexOf per comparison

Complexity reduction

Component	Before	After
DefaultGraphBuilder sort	O(n² log n)	O(n log n)
DefaultModelObjectPool.process()	O(k) stream + alloc per call	O(1) Set.contains
PoolKey.dependencyHashCode	1 Object[12] alloc per call	0 allocations
PhaseComparator.compare	O(n) per comparison	O(1) per comparison

Test plan

• DefaultModelObjectPoolTest passes
• Full maven-impl test suite passes
• DefaultGraphBuilderTest (blocked by pre-existing ProjectBuildLogAppender compile error on master — unrelated to this PR)
• CI validation

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