Implement speech bubble, narration, and SFX overlay tools

[realproject7]

Parent: #197

Proposal reference: ~/z-docs/PROPOSAL-plotlink-ows-webtoon-pivot.md

Context

The lettering editor must support actual webtoon text placement: speech bubbles, narration boxes, and SFX/display text.

Scope

• Add overlay creation tools for speech bubble, narration box, and SFX/plain text.
• Support editing overlay text.
• Support drag, resize, delete.
• Support basic bubble tail anchor adjustment.
• Persist overlays to plot-NN.cuts.json.
• Keep preset count small for MVP.

Acceptance Criteria

• User can add/edit/delete speech, narration, and SFX overlays.
• Overlay changes persist and reload.
• Bubble tail data persists.
• Tests cover overlay CRUD and persistence.

Public Safety

This is a public repo. Do not include secrets, API keys, wallet private keys, mnemonics, auth tokens, OWS vault internals, private story drafts, or sensitive local paths in issues, PRs, logs, docs, tests, or examples.