Docs: update CHANGELOG, TROUBLESHOOTING, README for merged CPU optimiser + Link + black-popup work

[shibco]

Track the documentation review for work merged after v2026.08.26.1 through 59c13ffc on main.

Release comparison: v2026.08.26.1...59c13ff

Review inventory

• Restore offscreen client content after an X11 expose #267 Restore offscreen client content after an X11 expose
• Ableton Link hardening and troubleshooting docs #268 Ableton Link hardening and troubleshooting docs
• [Moonshot PR] CPU optimiser pipeasio hotpath #269 PipeASIO output hot path
• [Moonshot PR] CPU optimiser research consolidation #270 CPU optimisation research consolidation
• [Moonshot PR] CPU optimiser bench reporting tool #271 CPU and audio benchmark reports
• [Moonshot PR] CPU optimiser hybrid topology #272 Hybrid CPU topology reporting
• [Moonshot PR] CPU optimiser pipeasio telemetry #273 PipeASIO callback telemetry
• [Moonshot PR] CPU optimiser pipewire arbitration #274 PipeWire quantum arbitration
• [Moonshot PR] CPU optimiser ntsync proof #275 NTSync runtime proof
• [Moonshot PR] CPU optimiser pipewire pro audio #276 PipeWire Pro Audio comparison tool
• [Moonshot PR] CPU optimiser wine bounded fastpaths #277 Wine bounded fast path trial
• [Moonshot PR] CPU optimiser live workers #278 Automatic Live audio worker count
• More hardening of the installer #279 Installer launcher replacement and recovery

Documentation tasks

• Confirm the latest release tag and the final merge commit.
• Review every pull request and direct commit in the release range.
• Consolidate user-visible changes in CHANGELOG.md.
• Add symptom and recovery guidance to TROUBLESHOOTING.md.
• Confirm the audience and scope for README.md with the maintainer.
• Update README.md from the agreed direction.
• Check document links, commands, headings, and release coverage.
• Review the final diff and close this tracker.

The local documentation work starts under 2026.08.26.2 — in progress.