Optimize model building pipeline: defer Dependency.build() and reduce allocations

[gnodet]

Summary

• Add Builder getters to generated model classes (model.vm) — enables reading builder fields through the base chain without calling build(), supporting deferred materialization across pipeline stages
• Add *ToBuilder merger variants (merger.vm) — returns Builder instead of calling build(), letting callers accumulate changes across multiple merge passes without intermediate allocations
• Defer build() in DefaultDependencyManagementInjector — accumulates merged dependencies as Builder objects, calling build() only once per modified dependency at the end of the loop
• Optimize computeLocations() — replaces Stream.concat().collect(Collectors.toUnmodifiableMap(...)) with HashMap.putAll() + Map.copyOf(), and returns oldlocs directly when newlocs is empty (avoids unnecessary Map.copyOf since the base map is already immutable)
• Precompute locationsHashCode — cached at build time in the generated constructor, used by DefaultModelObjectPool.PoolKey for fast inequality checks before full map comparison
• Optimize PoolKey.locationsEqual() — uses direct getLocations() map comparison instead of iterating individual keys, with fast-path for both-empty maps (common for imported deps)
• Add addLocationInformation API to XmlReaderRequest — wired through DefaultModelXmlFactory for future use in skipping location tracking on imported BOM POMs

Context

JFR profiling on a 4,383-module reactor shows 37% CPU in ModelObjectPool (dependency interning), 18% in Dependency immutable builders, and 2.3 GB allocated in KeyValueHolder from computeLocations(). Each Dependency passes through ~7 pipeline stages, each calling build() which allocates a new immutable object + recomputes location maps + calls processObject().

This PR targets the three hottest code paths:

1. Redundant build() calls — deferred via Builder getters and ToBuilder merger variants
2. computeLocations() stream overhead — replaced with HashMap merge + early return
3. PoolKey location comparison — precomputed hash + direct map equality

Test plan

• mvn verify -B passes across all reactor modules (all tests green)
• FileToRawModelMergerTest updated to exclude new *ToBuilder methods from reflection-based override check
• Benchmark with JFR on large reactor to measure allocation reduction

🤖 Generated with Claude Code