feat: add dialogue text speed override

[han4wluc]

Summary

• add dialogue.textSpeed as a one-line reveal speed override that falls back to runtime.dialogueTextSpeed
• expose the effective speed in dialogue template data, including NVL dialogue lines
• document speed: ${dialogue.textSpeed}, bump package version to 1.26.0, and add VT coverage/reference images

Verification

• bun vitest run spec/projectDataSchema.test.js spec/RouteEngine.layoutDialogueTemplates.test.js
• bun vitest run puty.spec.js -t "dialogue textSpeed"
• rtgl vt screenshot --wait-event vt:ready --item dialogue/text-speed
• rtgl vt report --item dialogue/text-speed
• bun run lint
• bun run test
• bun run build