Add doc comments for all public API and catalog Player/Party

[gwillish]

Summary

Documentation pass across the full public API surface:

• DaggerheartModels.docc/DaggerheartModels.md — adds a Players and Parties topic group with Player and Party so they appear in the generated DocC catalog
• Player.swift — property docs for all 8 stat fields; init with full Parameters block
• Party.swift — id and name property docs; init with Parameters block
• AdversaryState.swift — id, maxHP, maxStress, currentHP, currentStress, isDefeated, conditions docs; init with full Parameters block
• PlayerState.swift — id, name, maxHP, currentHP, maxStress, currentStress, conditions docs; init with full Parameters block
• EnvironmentState.swift — id doc; init with Parameters block
• EncounterSession.swift — id, name, gmNotes docs; init summary; incrementFear, spendFear, incrementHope, spendHope docs
• Compendium.swift — fileNotFound and decodingFailed case docs
• SessionRegistry.swift — sessions property doc; init summary
• EncounterStore.swift — notFound, saveFailed, deleteFailed case docs; init with Parameters block

No functional changes — doc comments only.

Test plan

• swift build passes (all targets)
• swift test — 162 tests pass
• ./Scripts/format.sh exits clean
• Linux CI passes