Implement the native Go dialogue and interface engine

[zoeyrose]

Outcome

Replace runtime Python interface construction with native Go conversation state and structured, bounded presentation contracts.

Scope and invariants

• Define stable conversation/topic/response IDs, speaker identity, entry/fallback resolution, session state, pagination, and cancellation.
• Compile authored text and response structure; evaluate visibility/enablement through typed service facts or approved pure CEL.
• Dispatch selected responses through typed gameplay commands with authorization, idempotency, and atomic effects.
• Represent text, semantic emphasis, links/actions, portraits, and input prompts as a renderer-neutral protocol model with escaping/localization boundaries.
• Bound text, choices, nesting, session lifetime, transitions, and response/action recursion; stale selections fail coherently.

Acceptance criteria

• Tests cover branching, missing topics, disabled/hidden choices, stale/replayed selections, disconnect/reconnect, escaping, pagination, transaction failure, and content revision.
• One NPC dialogue and quest handoff run in the M3 slice with zero Python/Starlark.
• The 83 current interfaces are present in the migration manifest with an engine feature/data/handler owner.
• The Rust client renders and automates the same structured model; no server HTML/legacy markup parser is required.
• Malformed authored definitions are rejected before catalog publication with source diagnostics.

Dependencies and parallelization

Depends on #36, #41, #43 where quest actions intersect, and #45 for approved conditions. Client UI and editor panels proceed against golden models.

Independent implementation

New code and tests are MIT-licensed independent work. Preserve public behavior and issue-level design choices, but do not copy or mechanically translate GPL server/Python implementation or tests. Mixed-license content remains external data with its own notices.