graph-lite: public header review before the first tag

[sebyx07]

graph-lite.h is a public contract: everything exported becomes a promise the moment anyone embeds it. Review the surface once, deliberately, before graph-lite-v0.1.0.

Done when

• Every exported symbol is intentional — nothing leaked from an implementation detail.
• No consumer-domain vocabulary anywhere in the header, its comments included.
• The standalone build and the embedding instructions in graph-lite/README.md both work from a clean checkout of just that directory.
• Version macros and graph_lite::version() agree, and a header/binary mismatch is detectable at startup.
• CHANGELOG.md has a graph-lite entry.