docs: make root README user-facing

[swgiacomelli]

Summary

• lead with the user value and the target/host crate choice
• add installation, a compile-checked typed conversion, and runnable examples
• make features, numeric guarantees, scope, compatibility, and verification easy to scan
• retain crates.io and docs.rs badges using their post-publication URLs

Documentation authority

Reviewed against Photon Circus Repository Standards at edde2dd4497183da168f56297ba82f381c86e0cc.

Verification

• exact quick-start snippet compiled and passed as a temporary integration test
• all three documented example commands ran successfully
• all repository-relative links resolve
• cargo xtask ci: all runnable checks passed, no skips

Live publishing state

Both crates are available on crates.io. The docs.rs endpoints are still queued and currently return 404; the canonical badge and documentation URLs are already in place and will resolve when docs.rs finishes the builds.