Run native Xcode script phases with reconciled build settings

Once now retains Xcode script phases without source or archive outputs, including validation and product-processing scripts. Preparation runs before compilation, resource generators can run between linking and packaging, and product scripts run after assembly and before final signing. Later scripts consume the bundle produced by earlier scripts, including undeclared resources created by always-run phases. Deleted resources stay deleted, and restoring a captured bundle no longer overwrites newer outputs inside it.

Scripts receive native build settings mapped to Once's workspace, product, and intermediate directories, including input and output file-list variables. Fully declared scripts can reuse cached results. Always-run scripts and scripts with untracked inputs bypass the cache while allowing individual compilation actions to remain cached. Installation-only phases remain excluded from ordinary builds.

Native shell phases execute in the workspace so absolute build-setting paths and in-place edits address the same products. Caching relies on complete input and output declarations, not filesystem isolation.