fix: restore RIRU skin on Codex 26.730

[CHOCK1337]

Summary

• support the Codex 26.730+ data-app-shell-* shell markers while preserving legacy class support
• use one :is(...) selector contract for renderer probes and generated CSS so selector unions cannot escape the skin root
• bump the cross-platform runtime to 1.6.1 and document the RIRU v1.2.1 hotfix

Verification

• Windows full PowerShell regression suite: pass (fresh local run, exit 0)
• Windows selector/renderer regressions: 18/18 pass
• Windows-executable macOS/shared Node contracts: 50/50 pass
• GitHub CI: all 4 jobs pass, including Windows PowerShell 5.1/7 and macOS regressions
• renderer benchmark: 3.3 ms median refresh, below the 50 ms budget
• Setup v1.6.1 built with the pinned Node runtime; version 1.6.1.0 and release hash verified
• staged privacy scan: no account names, session data, local paths, screenshots, or paid RIRU source assets

Live Codex 26.730 smoke

• launched a second Store-installed Codex 26.730.8199.0 instance with isolated AppData, Chromium profile, and loopback CDP port; the already-running user task was not restarted
• injected riru-moonroom runtime 1.6.1; shell, sidebar, header, and main markers all passed against the real renderer
• visually checked the full Win32 window after replacing 108 visible text nodes with privacy placeholders: the continuous RIRU background, character face/ears/tail, real sidebar, header, home cards, and composer rendered without a black surface or horizontal overflow
• the local diagnostic screenshot is intentionally not uploaded or committed; the repository showcase continues to use completely fictional demo data
• isolated processes, profiles, and CDP port were removed after the smoke test; the original Codex process set was preserved exactly