Add usage guide with practical examples and integration patterns

[yuribodo]

Summary

Comprehensive USAGE.md addressing the gap identified in #8: "demos look amazing, but unclear how to use without poking through the code."

The guide is organized by what you want to accomplish, not by function name:

• Quickstart — 3 lines to measure text height
• Choosing Your API — decision tree: layout() vs layoutWithLines() vs walkLineRanges() vs layoutNextLine()
• Core Concepts — two-phase model, font strings, cursors
• Height Prediction — vanilla JS + React hook with ResizeObserver
• Canvas Rendering — layoutWithLines() → ctx.fillText() loop
• Shrink-Wrapping — binary search pattern from the Bubbles demo
• Variable-Width Layout — text around images + two-column flow with cursors
• Pre-Wrap Mode — { whiteSpace: 'pre-wrap' } for textareas
• Performance — what to cache, when to re-prepare, batch patterns
• Framework Integration — React, vanilla JS, and a note for Vue/Svelte/Angular
• Tips — named fonts, font loading, lineHeight, edge cases

Does not duplicate the README's API glossary — references it instead. Added a one-line link from the README.

Closes #8

Test plan

• bun run check passes
• bun test passes (60/60)
• All internal links resolve (README, DEVELOPMENT.md)
• Code examples are self-contained and copy-pasteable
• Review examples for accuracy against current API