docs(release): prep v1.6.6 — Central install snippet + release status + community links (PR-1.6.6)

[DemchaAV]

Summary

Final pre-cut PR from the readiness taskboard. Brings README + CHANGELOG up to the shape v1.6.6 will ship in. After this merges, graphcompose-release-engineer cuts v1.6.6.

What changes

README

Where	What
Hero badges	New Maven Central badge alongside JitPack.
New "Release status" block	🟢 Latest stable v1.6.5 (JitPack) · 🟡 In develop v1.6.6 (Maven Central debut; zero breaking) · ⚪ Planned next v1.6.7 (dep cleanup), v1.7.0 (new DSL primitives). Links to docs/api-stability.md for tier definitions.
Install section	Maven Central (io.github.demchaav:graphcompose:1.6.6) listed first as the primary path. JitPack snippet stays as fallback / pre-v1.6.6 alternative.
Footer "Contributing & releases"	New links to CODE_OF_CONDUCT.md, SECURITY.md, docs/api-stability.md, docs/templates/which-template-system.md — files that were committed long ago but never linked from the README.

CHANGELOG

v1.6.6 — Planned summary expanded to three paragraphs:

• What shipped — Central, sources/javadoc jars, GPG, japicmp gate, docs polish.
• Zero breaking changes from v1.6.5 — existing API surface unchanged (validated by japicmp against v1.6.5 baseline).
• Migration from v1.6.5 — "no code changes required; optionally swap to the Central coordinates."

Version-pinning intent

Snippet	Pinned at	Why
JitPack <dependency>	v1.6.5 (current pom)	cut-release.ps1 flips this to v1.6.6 at release time, matching the VersionConsistencyGuardTest invariant (README JitPack snippet == pom version).
Central <dependency>	1.6.6 (explicit)	v1.6.5 was never on Central, so pinning to current pom would advertise a non-existent artefact. VersionConsistencyGuardTest only scans the JitPack snippet (matches <artifactId>GraphCompose</artifactId> — capital G; Central uses lowercase graphcompose), so the Central pin is exempt from the guard. Maintainer manually bumps this in each subsequent release-prep PR.

Verification

$ ./mvnw -B -ntp test -pl . -Dtest='CanonicalSurfaceGuardTest,DocumentationCoverageTest,VersionConsistencyGuardTest,DocumentationExamplesTest'
Tests run: 25, Failures: 0, Errors: 0, Skipped: 0

$ ./mvnw -B -ntp test -pl .
Tests run: 1033, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS (~91s)

Doc guards green, full suite green.

What's next (post-merge)

1. Maintainer wires the four GitHub repo secrets (MAVEN_GPG_PRIVATE_KEY, MAVEN_GPG_PASSPHRASE, CENTRAL_USERNAME, CENTRAL_TOKEN) per docs/contributing/release-process.md § 2.C — one-time setup.
2. Cut v1.6.6 via graphcompose-release-engineer skill: cut-release.ps1 bumps the four pom version sites + the JitPack snippet to 1.6.6, dates the CHANGELOG, runs verify, commits, tags v1.6.6, pushes.
3. Three workflows fire on the tag push, in parallel:
   • release.yml — creates the GitHub Release with the CHANGELOG section as the body.
   • publish.yml — signs + uploads to Maven Central; the maintainer flips the "release" switch on central.sonatype.com after Central's validator confirms.
   • JitPack picks up the tag on first request.
4. javadoc.io/doc/io.github.demchaav/graphcompose/1.6.6 becomes available within minutes of the Central publish.

Test plan

• All doc guards green
• Full suite green
• VersionConsistencyGuardTest passes (JitPack snippet matches pom; Central snippet exempt by artifactId difference)
• CI green on PR (Guards / JDK 17/21/25 / Examples Smoke / no-poi-suite / binary-compat)
• Reviewer skim — README install + release-status block + CHANGELOG summary are the load-bearing changes
• (Out of scope but called out) follow-up: extend VersionConsistencyGuardTest to also enforce the Central snippet's 1.6.6 matches the pom — once Central versioning has shipped a few times and we know the bump cadence we want