Add ordered flashcard stages

[backslash-f]

Summary

• add stable host-authored stage identities and ordered prompt/answer stages
• validate nonempty stage sequences and reject duplicate stage identities during construction and decoding
• preserve the existing simple-card initializer and first-stage prompt/answer accessors
• preserve original single-card Codable interoperability while encoding the new staged representation
• document staged cards and keep the existing ThreeChoiceSession behavior on each card's first stage
• sync AgentGuidelines 0.0.34 with exact tagged-tree provenance

Validation

• AgentGuidelines/Scripts/swift_format.sh format-and-lint Package.swift Sources Tests
• AgentGuidelines/Scripts/swift_format.sh lint-strict Package.swift Sources Tests
• swift build
• swift test — 28 tests / 35 parameterized executions, zero failures
• AgentGuidelines/Scripts/validate_consumer_setup.swift --consumer-root . --require-swift-format
• AgentGuidelines/Scripts/validate_guidelines.swift AgentGuidelines
• swift package dump-package — Swift 6 language mode plus warning/error and required upcoming-feature settings verified for library and test targets
• .agents/skills/agent-guidelines-audit/scripts/check_markdown_wrapping.swift README.md AGENTS.md Sources/FlashcardKit/FlashcardKit.docc
• git diff --check
• AgentGuidelines subtree tree b40e2c75de8681ade6a5ae97ad3ddd8a28f3adda exactly matches tag 0.0.34

Audit notes

• No dependency graph changes.
• The new types are pure validated values with no lifecycle or operational boundary, so they intentionally emit no runtime logs.
• FlashcardKit's existing 🃏 registry identity, AppLogger linkage, and com.thatfactory.flashcardkit subsystem remain consistent.