Export homebrew adversaries as a .dhpack file

[gwillish]

Summary

Allow users to select adversaries they have added to the app (homebrew/local content) and export them as a .dhpack file for sharing with other Encounter users.

Motivation

The app already supports importing .dhpack files. Closing the loop with an export flow makes it easy for GMs to build a pack inside the app and share it — without needing a text editor or knowledge of the JSON schema.

Work to scope (UX TBD)

The exact UX needs a dedicated design session, but the high-level flow is:

• User selects one or more homebrew adversaries (and optionally environments) from the Compendium browser or a dedicated export screen
• App serializes the selection into the pack-wrapper JSON format ({ "adversaries": [...], "environments": [...] })
• Standard system share sheet presents the .dhpack file — user can AirDrop, save to Files, send via Messages, etc.

Open UX questions

• Where does export live in the nav? (Compendium browser toolbar, long-press context menu, multi-select mode, dedicated "My Packs" screen?)
• Should users be able to name the exported pack before sharing?
• Should the export include a source field auto-populated with the user's name or a pack name they provide?
• Should it be possible to export a mix of homebrew and SRD adversaries, or homebrew only?
• Should exported packs be saveable as named drafts within the app before sharing?

Related

• Depends on Docs: .dhpack format — how to create and share custom adversary packs #15 (.dhpack format documentation) — AuthoringAPack.md will mention in-app export as one authoring path
• See schemas/dhpack.schema.json for the format the export must conform to
• Extract model types into a Swift Package (enables SPI docs hosting + validate-dhpack CLI) #19 (Swift Package extraction) — the serialization logic belongs in the model layer and should be covered by the package's validate-dhpack CLI