[codex] fix 26.727 shell selector without broad target fallback

[alucard210319]

Summary

• update the canonical shell-main selector to support both the legacy main-surface class and the stable data-app-shell-main-surface attribute introduced in Codex 26.727
• regenerate the synchronized macOS and Windows CSS, selector, and renderer assets
• add cross-platform regressions for the current shell and for rejecting generic auxiliary app targets

Root cause

The signed macOS Codex 26.727.40816 bundle no longer gives the main surface the literal main-surface class. It uses a CSS-module class and exposes data-app-shell-main-surface="default" instead. The v1.5.9 probe therefore fails before injection even though the sidebar and other Codex identity anchors are still present.

This patch keeps one composable selector:

main:is(.main-surface, [data-app-shell-main-surface])

Using a single selector also preserves existing CSS :has() composition. The target identity boundary remains shell plus native sidebar; this intentionally does not add a generic main/input or text-branding fallback, which could attach to an auxiliary app renderer.

Validation

• synchronized runtime assets check
• selector doctor and renderer/bootstrap/readiness regressions
• nested :has() CSS regression: 6/6
• Windows portable Node suite: 18/18
• shell and Node syntax, payload, version, encoding, runtime-safety, and diff checks
• GitNexus impact: LOW, limited to the existing one-shot/watch probe paths

The local full macOS runner reaches an unrelated native Swift build and stops because this machine has a Swift 6.3.2 SDK with a Swift 6.3.3 compiler. The isolated Codex launch also exited at the single-instance boundary before the main UI loaded, so the upstream macOS CI and a maintainer smoke test remain authoritative for native/full-UI verification.

This is a focused alternative for #322 only; it does not include the #318 or #320 changes currently grouped in #324.

Fixes #322