Feature: Custom GC, Packaging + Doc Improvements

[Eli Pinkerton (wallstop)]

Description

This PR resolves memory-retention issues in DxMessaging and improves long-run stability.

User-facing impact:

• Reclaims stale message-handler registrations so long-running sessions do not keep growing memory.
• Keeps dispatch behavior stable while reducing pressure from orphaned slots/links.
• Adds runtime settings so Unity projects can control reclamation behavior without code changes.
• Expands diagnostics and documentation for troubleshooting and migration.

Related Issue

Fixes #

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor (code change that neither fixes a bug nor adds a feature)

Checklist

• All tests pass locally
• Code is properly formatted
• I have added tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have updated the CHANGELOG
• My changes do not introduce breaking changes, or breaking changes are documented

Performance numbers

Scenario	Baseline run	This PR	Delta
Dispatch throughput (Mono Editor)	Captured in baseline benchmark run	Captured in this PR benchmark run	Reported by CI
Dispatch throughput (IL2CPP Player)	Captured in baseline benchmark run	Captured in this PR benchmark run	Reported by CI

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