Changelist: complete the 2.0.0 release notes

[drowaudio]

The release job publishes CHANGELIST.md as the release notes, and nothing has been released since v1.0.4, so 2.0.0 is the first release of everything on master since then. The 2.0.0 section was missing several of those changes.

What

• Added missing entries:
  • Real-time safety checking on macOS via rtcheck (--rtcheck, off by default) [rtcheck - real-time safety checking #156]
  • The built-in VST3 validator (runs for VST3 plugins at strictness 5+) [Integrate VST3 validator directly into pluginval binary #162], and the removal of --vst3validator, listed as Breaking since passing it is now an error
  • The user-visible fixes from Fix timeout races and other issues from the v2 review #198: timeout/report races, garbled child-process output, --output-filename resolution, and the operator new / operator delete replacements
• Folded in 1.0.5: the 1.0.5 section (drag and drop, static Windows runtime, PluginInfoTest on the message thread) was never tagged or released, so its entries are now part of 2.0.0 and the 1.0.5 heading is gone.
• Grouped the 2.0.0 entries under New / Changes / Breaking / Fixes, with a short "Upgrading from 1.x" note at the top, since CI scripts that download releases/latest will move from 1.0.4 to 2.0.0 automatically.

Every existing entry is kept word for word; the three "Breaking:" items lost their prefix because they now sit under the Breaking heading.

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