docs+chore: README + showcase Maven-Central-only; guard accepts Planned version (Phase 2)

[DemchaAV]

Summary

Phase 2 of the Maven Central coordinate migration: forward-facing install snippets in `README.md` and `docs/index.html` now show only Maven Central coordinates; the legacy JitPack URL stays alive for v1.6.5-and-earlier pinned consumers but is no longer documented as an install option anywhere.

The chicken-and-egg this PR resolves: the install snippet has to advertise the about-to-ship version (1.6.6) so users who copy it get a coord that resolves once the tag lands, but the library `pom.xml` still carries the previous version (1.6.5) until cut-release runs.

The fix is in `VersionConsistencyGuardTest`: the snippet version must equal either the current `pom.xml` version OR the version named in the top `## v<X.Y.Z> — Planned` CHANGELOG entry. `cut-release.ps1` bumps both to the same target in the release commit, after which the two converge and the looser check continues to pass.

What changes

`README.md`

• JitPack install section deleted (was both Maven + Gradle blocks, lines 111-128)
• JitPack `img.shields.io` badge deleted
• `### Maven Central (primary, from v1.6.6)` section header demoted to plain `## Installation` since Central is the only option now
• Distribution status note rewritten: Central is canonical, JitPack remains resolvable for legacy pinned consumers but is not a documented install path
• "Latest stable" status block drops the "(JitPack)" qualifier

`docs/index.html`

• Install section H2: `JitPack, no extra setup.` → `Maven Central. One dependency.`
• Maven install block: drops `` block, switches groupId to `io.github.demchaav`, artifactId to lowercase `graphcompose`, version drops the v-prefix
• Gradle install block: drops `repositories { maven }` block, switches coord to `io.github.demchaav:graphcompose:1.6.6`
• JSON-LD `downloadUrl` switches from `https://jitpack.io/...\` to `https://central.sonatype.com/artifact/io.github.demchaav/graphcompose/1.6.6\`

`VersionConsistencyGuardTest`

• `readmeInstallSnippetsMatchTheProjectVersion`: accepts either pom version or CHANGELOG Planned version (set-membership instead of strict equality)
• `showcaseSiteVersionMatchesTheProjectVersion`: drops the JitPack `downloadUrl` assertion, uses Maven Central regex for the install snippets, also accepts the pom-or-Planned target set
• Both snippet checks try a Maven Central regex first, fall back to the legacy JitPack regex — handles the snippet during any transition window cleanly
• Pulls Planned version from CHANGELOG via regex on `^## v(...) — Planned`

`scripts/cut-release.ps1`

• `Update-ReadmeVersion` bumps Maven Central snippet (lowercase artifactId, bare semver) with a fallback to legacy JitPack regex
• `Update-IndexHtmlVersion` drops the JitPack downloadUrl regex, adds Central artefact URL regex, switches install-snippet regexes to Maven Central format

Not changed (intentional)

Sample CV data in `ExampleDataFactory.java` and `CvV2VisualParityTest.java` still lists "JitPack" as a fictional skill. That string is fictional CV content (a fake person's listed skill), not a documented coord. Changing it would diff every CV-v2 visual snapshot baseline (16 presets) for a cosmetic gain.

Test plan

• `./mvnw test -Dtest=CanonicalSurfaceGuardTest,DocumentationExamplesTest,DocumentationCoverageTest,VersionConsistencyGuardTest -pl .` → 30/0/0
• CI: standard guard suite + JDK 17/21/25 + binary-compat + examples smoke + perf smoke

Stack

Independent from #102 (EngineShowcase) and #103 (Phase 1 non-version-locked docs). All three target `develop`, can land in any order.

After all three merge, the migration is complete — `develop` carries only Maven Central documentation, the install snippet shows the v1.6.6 coord, and `cut-release.ps1` is ready to bump it on tag cut.