Support codex-app Linux layouts and version-safe backups

[siraht]

Summary

Add installer compatibility for the community Linux packages from
am-will/codex-app, including the current ChatGPT-branded Codex payload, and keep the
Codex++ rollback backup aligned with the app build being patched.

Changes

• Detect common codex-desktop Linux install roots under /usr/lib, /opt,
  ~/.local/opt, and ~/.local/share.
• Resolve launcher/current-version symlinks before determining the app root.
• Accept the Linux Electron executable names Codex, codex-desktop, and codex.
• Search for main-*.js beside a nested recovered entrypoint as well as the standard
  .vite/build directory.
• Match the current window-services factory when appBrand appears before
  buildFlavor in its options object.
• Rotate the backup set when the app root or in-place app build changes.
• Preserve a matching pristine backup on repeated installs of the same patched app.
• Reject an already-patched app when its saved state and pristine backup do not agree,
  instead of creating an unsafe rollback point.
• Abort before rotating backups when target app metadata cannot be read.

Why

The Linux port uses layouts such as:

/usr/lib/codex-desktop/Codex
/usr/lib/codex-desktop/resources/app.asar
~/.local/opt/codex-desktop/current -> <versioned install>

Those layouts were not covered by the existing Linux locator. The current 26.707
ChatGPT/Codex payload also constructs window services with appBrand before
buildFlavor, which caused installation to fail with:

Codex window services hook point not found

Separately, Codex++ uses one backup directory. Keeping the first-ever app.asar
indefinitely means that installing against a newer app can leave uninstall prepared
to restore an unrelated old build. The new backup preparation step validates repeated
installs and retires the old set before backing up a fresh app build.

Validation

npm test
npm run build
git diff --check

The test suite covers:

• directory and symlink Linux overrides
• nested recovered main-bundle discovery
• appBrand before buildFlavor
• new install-root backup rotation
• in-place app update backup rotation
• repeated-install backup preservation
• stale backup rejection
• invalid target metadata preservation

Live validation against a user-owned am-will/codex-app 26.707.31123 package completed
installation, patched .vite/build/early-bootstrap.js, flipped the Electron integrity
fuse, and passed codexplusplus status and codexplusplus doctor.