refactor: organize Workshop internals by domain

[e54-bot]

Summary

• organize source around discoverable Workshop domains: actions, events, rules, values, settings, gameplay, and catalog
• isolate shared frontend, analysis, output, core, and evidence implementation trees
• split WIR domain models and preserve legacy public module paths with re-exports
• document the source layout and ownership boundaries

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets
• cargo run -p workshop-rs --bin workshop-catalog-gen -- check
• cargo doc --workspace --no-deps

Fixes #149

[Teakowa]

Blocker — frontend/parser.rs and output/emitter.rs: #149 requires domain/feature-first organization and explicitly allows shared parser/emitter state only as cross-domain orchestration/context, not as the semantic home of all features. This PR mostly moves the existing parser and emitter wholesale into phase-wide frontend//output/, while actions.rs, events.rs, rules.rs, and values.rs are thin re-export facades. Contributors still have to search the phase-wide parser/emitter to understand action/event/value behavior, so the central acceptance criterion is not met. Move feature-owned parsing/emission/related behavior under the relevant domain modules and leave only genuinely cross-domain orchestration/context in the shared boundaries.

[Teakowa]

LGTM