docs: rewrite README to showcase actual capabilities

Lead with what makes fingerprint different: corpus-based infer mode,
chained parent/child fingerprints, diagnostic context on failures,
26 assertion types, and DSL-to-Rust compilation. Move technical
contract details to the back half. Surface undersold features like
schema-driven infer, frankensearch hybrid search, markdown
normalization, and real-world CMBS examples.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Zac