v0.3.0 — sane default for step reveals

Changed (BREAKING)

• Reveal-step semantics flipped. A slide is now one reveal step by default — every block appears at once — instead of one step per block. Authors who want progressive reveal put +++ on its own line where they want the break. Leading, trailing, and consecutive +++s are elided. +++ inside a fenced code block is literal text.
• Scene-contract data shape. A slide is now Array<Array<Block>> (outer = reveal steps, inner = blocks shown together). The runtime (content-slide's renderSlide), the linter's per-block dispatch (bin/talk-lint.js), and the Vite plugin's collectSceneDiagnostics all updated for the extra nesting level. Any external code that iterated slide.map(block => …) now needs a second loop.

Example

Before — three blocks, three clicks to reveal:

# Principles
A short intro.
- bullet one
- bullet two

After — same markdown, one click reveals the whole slide:

# Principles
A short intro.
- bullet one
- bullet two

Want the old progressive reveal? Add +++:

# Principles

+++

A short intro.

+++

- bullet one
- bullet two