Tighten README around core use case

[unitmatrix]

Summary

• shorten repeated explanations of Range behavior, bounded-memory lookup, and output semantics
• move the C library API earlier so the reusable interface is visible sooner
• use the actual CLI write_file() / zget_get() path as the canonical extraction example
• keep HTTP, scope, build, packaging, and pre-1.0 compatibility details in one place each

No code or behavior changes.