feat: make split-flap-elements npm-ready

[gabeosx]

Summary

• recalibrates default motion around a slower 140 ms flap cadence, 1.4 s minimum spin, reel-order movement, randomized starts, concurrent spinning, and naturally grouped settling
• fixes frame and cell validation, autoplay-after-assignment, pause/resume, programmatic reels, listener cleanup, default spans, and accessible cell/announcement output
• adds version-pinned direct-browser output to the builder, preserves bundler output, and refreshes the live samples, README GIF, social card, and share metadata from the real demo
• tests runtime and TypeScript consumption from the exact npm tarball and ships a Custom Elements Manifest
• hardens Release Please CI without a PAT, protects main and release tags, enables private vulnerability reporting, and documents the exact first-publish and Trusted Publishing flow

Validation

• npm run validate
  • 28 unit tests
  • 38 passing browser/visual tests across Chromium, Firefox, and WebKit
  • 4 expected non-Chromium visual snapshot skips
• npm audit --audit-level=low (0 vulnerabilities)
• actual 33-file npm .tgz installed, type-checked, and browser-smoke-tested in a clean fixture
• desktop and 390 px mobile review of the demo, preset gallery, builder, and generated import variants

Release gates

This PR does not publish to npm, enable NPM_PUBLISH_ENABLED, merge the Release Please PR, or rewrite published history. Release Please PR #5 remains an explicit release decision.