Implement Swift Protocol v1 conformance and platform-specific authoring

[JacobLinCool]

Renamed by #23. This issue was written while the project was called Android Data
Collector. Its terminology has been updated to the Particeps identity now in force. The
old spellings are rejected inputs, not an earlier dialect.

Summary

Implement a standalone Swift package for the final Particeps Protocol v1 and add explicit Android/iOS targeting to researcher authoring.

Android and iOS may share an experiment_id, but they must use separately signed artifacts with unique configuration_id values, platform-specific collector scope, consent, access requirements, and minimum client version. Do not use silent capability negotiation or make a Kotlin Multiplatform rewrite a prerequisite.

Requirements

Swift Protocol v1 package

• Implement strict Protocol v1 configuration and export types without importing Kotlin/JVM or Tink wire representations.
• Use CryptoKit for:
  • Ed25519 signing and verification through Curve25519.Signing;
  • raw X25519 key handling;
  • RFC 9180 HPKE with the exact X25519/HKDF-SHA-256/AES-256-GCM suite, mode, context, and constants defined by Finalize a language-neutral Particeps Protocol v1 and conformance corpus #8;
  • AES-GCM content encryption/decryption where specified.
• Encode Ed25519/X25519 public keys as unpadded base64url raw 32-byte values.
• Implement RFC 8785 canonical JSON, strict decimal-string 64-bit values, integer physical-unit configuration, complete bounds, duplicate/unknown-field behavior, frame parsing, and fail-closed legacy-v1 rejection.
• Consume the exact shared valid/hostile conformance corpus from Finalize a language-neutral Particeps Protocol v1 and conformance corpus #8; do not translate fixtures into Swift-specific equivalents.
• Expose typed APIs for configuration verification, bundle encryption/decryption, receipt validation, registry/capability lookup, and stable error classification.
• Avoid library-default algorithm identifiers or contexts that could diverge from the normative wire contract.

Cross-language conformance

• Verify every shared canonicalization, signature, digest, HPKE, framing, receipt, and hostile-input vector.
• Add Android/Kotlin → Swift decrypt/verify tests.
• Add Swift → Kotlin and Swift → Python decrypt/verify tests.
• Include wrong-key, wrong-context, wrong-configuration, malformed-frame, trailing-byte, Unicode, numeric-bound, and old-artifact rejection cases.
• Make these cross-language tests required CI gates.

Platform-specific researcher authoring

• Require the researcher to choose Android or iOS when creating a configuration.
• Permit a research project to reuse an experiment_id across platforms, but generate a fresh configuration_id and signature for every platform artifact.
• Bind the selected platform, minimum client version, collector availability, access requirements, intervention scope, data disclosure, and consent text into the signed artifact.
• Generate allowed collector/control choices and an Android/iOS parity matrix from Create a machine-readable collector and event-schema registry #10.
• Refuse to sign a configuration containing an unsupported required collector or platform feature.
• Do not silently omit, substitute, downgrade, or reinterpret a collector when changing targets.
• Make platform differences visible before signing and in exported provenance.

Acceptance criteria

• Swift passes the complete Finalize a language-neutral Particeps Protocol v1 and conformance corpus #8 valid and hostile corpus byte-for-byte.
• Android-produced bundles decrypt and verify in Swift; Swift-produced bundles decrypt and verify in Kotlin and Python.
• Raw key, signature, canonical JSON, HPKE context, framing, and receipt encodings have no Swift-only variant.
• Old Protocol v1/Tink artifacts and unsupported algorithms fail closed; no legacy fallback exists.
• Two platform configurations may share an experiment ID but cannot share a configuration ID or signature.
• Web authoring cannot sign an iOS configuration containing an Android-only required collector, or vice versa.
• Switching the authoring target never silently alters a previously signed artifact.
• The parity matrix, Web controls, Swift types, Kotlin types, and Python schemas agree with the same registry digest.
• The implementation does not require or begin a Kotlin Multiplatform rewrite.

Documentation

Add Swift package/API documentation, CryptoKit mapping, cross-language conformance instructions, platform-authoring guide, parity matrix, key-handling guidance, error taxonomy, and Protocol v1 implementation notes.

Reference: CryptoKit HPKE.

Dependencies

Blocked by #8 (Protocol v1), #9 (final exact-replay upload/receipt contract), and #10 (collector/event registry).