fix: isolate terminal resize consumers

[robfeldmann]

Summary

• give PlatformIO.sizeChanges and semantic input events independent terminal resize streams instead of competing iterators over one AsyncStream
• mark open-source readiness Phases 7 and 8 complete
• record anonymous repository, package-consumer, fork/vouch, CI, release-note, Pages, runner, and trust-only CI evidence

Failure reproduced

The public macOS job lost the second terminal-size notification because PlatformIO.sizeChanges and PlatformIO.events consumed the same unicast stream. The focused test failed at PlatformIOSizeTests.swift:58 on CI.

Validation

• focused size changes yields terminal size notifications test
• just quality format
• swift test (631 tests)
• just quality lint
• pnpx markdownlint-cli .agents/plans/031-open-source-readiness.md