docs(readme): fix stale Pages/CI URLs, currency pass, honest Compactness

Fixes the github.io/github.com/garnix URLs from the hyphen repo name to the
actual underscore (printable_binary) — the live-demo link and both CI badges 404'd
(command-name usages stay hyphenated). Rewrites the Web Interface section for the
new Encode/Decode tabs (paste-to-decode, .pbf.json container, metadata restore);
adds the container to Features; documents the previously-undocumented hexlike (-X)
mode; notes the test-fixture use case (inline binary literals) in the intro. Makes
the Compactness claim honest: ~1.2x for ASCII-heavy data, 2-3x for high-byte data.

Author: pmarreck