Add curl installer and visual product README

[jakthom]

Summary

• lead with the problem n2k solves and a complete debugging-workflow overview
• add a copy-paste curl installer with SHA-256 verification
• support macOS, Linux, and Windows across amd64 and arm64 release assets
• document Homebrew, go install, release archive, and source-build paths
• add TUI and scriptable CLI screenshots
• add a hardware-free quick start using the bundled sample capture
• exercise the release installer in the Linux, macOS, and Windows CI matrix

Verification

• installed the published release through the pushed curl pipeline
• verified all six release archive names
• validated both SVG assets
• ran the sample-capture command and Go tests
• checked shell syntax and README whitespace