docs: clarify Swift package support

[garethpaul]

Summary

• Document CocoaPods and direct Xcode integration as the currently verified package surfaces.
• State that Swift Package Manager is unsupported without a manifest, resource rules, and hosted build authority.
• Add a fail-closed package-boundary contract.

Baseline

Project documentation could imply Swift Package Manager support even though the repository has no Package.swift manifest or verified SPM resource/build configuration.

Improvements

• Describe only the CocoaPods and direct Xcode surfaces that current evidence verifies.
• Make the absence of Swift Package Manager support explicit.
• Prevent documentation from claiming SPM support until the required implementation and hosted authority exist.

Validation

• Two fresh detached clones of 4d13bc857c3fe32c12cb37e268d913ad2f70cb63 passed identical six-context static matrices.
• Geometry, image content mode, accessibility boundaries, podspec/Xcode metadata, and non-SPM contracts passed.
• Three exact-head hosted checks succeeded, including the macOS baseline simulator/sample-app authority.
• No IP network operations or Gitleaks findings occurred.

Risk

• This documentation/static-contract change intentionally narrows package claims and does not add Swift Package Manager support.
• Xcode, simulator, visual rendering, device signing, publication, uploads, and Apple services remain hosted or external boundaries.

Follow-Ups

• Add Swift Package Manager support only with a real manifest, resource rules, tests, documentation, and hosted macOS build authority.
• Keep verified installation surfaces synchronized with repository artifacts.