Support non-speaking characters in the scene character list

[stultus]

Area: src/lib/editor/characterList.ts, scene-cards data model, PDF export
Severity: Feature

Problem

The "Characters under scene heading" feature lists only characters who have a
`character` / dialogue node in that scene. A character who is present in the
scene but doesn't speak (mentioned only in action text) is invisible to the
collector and won't appear on the editor line or in the PDF.

Proposed solution (recommended)

Add a per-scene "Characters in scene" field to the scene annotation / scene
card (comma-separated). At render time:

1. Auto-detect speakers from `character` nodes (current behaviour).
2. Merge with the user-supplied list from the scene card.
3. De-duplicate, preserve user-supplied order first, render on the characters
   line.

Alternatives considered

• Markup convention in action text (`@Rahul`, ALL-CAPS tokens). Zero UI
  but unreliable for Malayalam names and easy to collide with ordinary prose.
• New schema element (non-speaking character / beat node). Clean but
  invasive — requires schema, keymap, and PDF changes.

Acceptance

• New field visible on the scene card and in the annotation gutter.
• Setting-aware: only collected when "Characters under scene heading" is on.
• Auto-populated speakers still appear without the user doing anything.
• Works for Malayalam names.