Schema: meta.extra for non-standard Fountain title-page keys

[stultus]

Part of #184 (Fountain import).

What

Add a meta.extra: Map<String, String> field to the .screenplay document schema, holding any title-page keys that don't map to first-class fields (title, author, director, tagline, registration_number, footnote, contact, draft_number, draft_date).

Why

Fountain title pages allow arbitrary keys (Source:, Notes:, Copyright:, plus anything writers invent). On import we have to put them somewhere; on Fountain re-export we have to emit them back. Without this field, a co-writer's custom title-page metadata silently disappears on round-trip.

Implementation sketch

• src-tauri/src/screenplay/document.rs — add pub extra: HashMap<String, String> to the meta struct, with #[serde(default, skip_serializing_if = \"HashMap::is_empty\")] so legacy files keep round-tripping byte-clean.
• BTreeMap instead of HashMap if we want stable key ordering on save (probably yes — diff-friendlier for git-tracked .screenplay files).
• No UI surface in v1. Keys are read-only inside Scriptty; only the importer writes them, only the exporter emits them.

Acceptance

• New field round-trips through serde without changes to existing fixtures.
• A .screenplay with no extra field loads identically to before.
• Loading then re-saving an old file does not introduce an empty extra: {} to the JSON.

[stultus]

Landed in c28e8e2 (schema) — round-trips through #186/#188.