[codex] fix Codex 26.727 Settings renderer detection

[Fei-Away]

Root cause

Codex 26.727 replaced the old Settings appearance anchors (appearance-theme
and theme-preview) with a settings navigation surface. When Settings was
open, both platform injectors rejected the real app:// target with
No page matched the expected ... shell markers.

Fix

• Register the stable data-settings-panel-slug="general-settings" marker in
  the shared selector contract.
• Use it consistently in macOS and Windows target identity, early injection,
  route classification, doctor output, and visible Settings verification.
• Keep the legacy Settings anchors and the strict app: origin gate.
• Add behavior tests proving non-app: targets and unbranded generic targets
  still fail closed.

Validation

• Real macOS Codex 26.727.40816: Home and thread remained L1; Settings now
  passes settings/L0 with the exact active community theme and no overflow.
• Shared runtime/asset sync: passed.
• macOS and Windows bootstrap/identity fixtures: passed.
• macOS and Windows renderer fixtures: passed.
• Selector doctor tests: passed.
• Full applicable macOS repository suite: passed; only the documented
  full-Xcode/installed Doctor branches skipped.
• git diff --check: passed.

Windows acceptance after release

Install the public Setup.exe, not a source-tree script. On current Codex:

1. Verify a full community theme on Home and an existing thread.
2. Open Settings on General and Appearance; Verify must not report a shell
   marker error.
3. Return to the app, restart Codex, and verify the same theme again.
4. Import two valid ZIPs with the same theme.json.id; the second must replace
   the first in place with one saved-theme directory.

Do not close #318, #320, #322, or #326 until the follow-up public Release is
installed and the macOS/Windows checks above pass.