Mobile: swipe navigation + scroll-instead-of-clip slides

[socalindc]

On phones the deck was hard to use two ways: a slide whose content runs taller than the viewport was clipped at both ends (the stage centers and hides overflow, so e.g. a 6-cell StatGrid rendered as a blank slide at 375×812), and the only way to advance was the small dock arrows.

Changes

• Swipe navigation (Deck.tsx): a quick horizontal swipe pages the deck, including <Build> reveals. Vertical drags are left free for slide scrolling; slow or diagonal drags don't page; touches on the dock, rail, grid view, presenter panel, annotator, and form controls are ignored. Listeners are passive.
• Scroll instead of clip (base.css ≤640px): slides become scroll containers with justify-content: safe center — short slides stay visually centered, tall ones scroll from the top, with bottom padding so the last content clears the dock. The atmosphere layer is pinned static there (its -15% bleed plus the drift animation's scale created ~140px of phantom scroll on every slide once slides could scroll).
• Split stacking (≤640px): the media pane keeps its natural height (minmax(min-content, 1fr)) instead of being crushed into the leftover row, and the slide scrolls when the stack runs long.
• StatGrid (≤640px): compacts to 2-up cards so six stats fit one phone screen (full-width stacking ran ~1.5 screens tall).
• Data tables (≤640px): fit the phone width — compact type, headers and label cells wrap (number strings carry no break points, so they stay whole) — instead of forcing a 540px min-width and panning sideways, which buried the right-hand columns. overflow-x: auto remains as the escape hatch for genuinely too-wide tables.
• Safe areas: dock and annotator bar respect env(safe-area-inset-bottom); viewport-fit=cover added to the viewport meta.
• SKILL.md: documents the phone behavior, per CONTRIBUTING.

Testing

• npx tsc --noEmit and npm run build both pass.
• Exercised at 375×812 and 375×660: every demo-style slide type either fits or scrolls correctly; swipe forward/back verified, vertical drags and slow drags correctly don't page.
• Desktop (>640px) rendering is unchanged — all changes live inside the existing ≤640px media queries plus the touch handler (inert without touch input).

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