docs: bump README install snippets to the current release version

[OmarAlJarrah]

Summary

• The README's Maven Central coordinate table, the Gradle and Maven install snippets, the Kuri.VERSION example, and the versioning-and-stability blurb all still pinned 0.1.0-alpha.1 — three prereleases behind the actual current 0.1.0-alpha.4 (per gradle.properties / .release-please-manifest.json). Anyone copy-pasting the install snippet as-is would pin a stale alpha and silently miss every fix shipped since.
• Bumped all five occurrences in README.md to 0.1.0-alpha.4.

Scope note

The issue also lists version pins in site/src/content/docs/guides/installation.mdx, site/src/pages/index.astro, and site/src/content/docs/guides/versioning-and-stability.mdx. None of those files exist on main yet — they're part of the still-open docs-site PR (#81) and aren't reachable from a main-based branch. That PR should pick up the same fix before or as part of merging; recommend wiring those specifically to the existing KURI_VERSION constant (kuri/build.gradle.kts:43-64) since the Astro/MDX build already has a code path that can inject a real generated value there, unlike plain README markdown.

For README.md itself, I looked at wiring the version through a release-please extra-files marker (the way gradle.properties already does) instead of hand-fixing it, but three of the five occurrences sit inside fenced Gradle/Maven/Java code blocks meant to be copied verbatim — the x-release-please-start-version/-end HTML-comment markers would render literally inside those copied snippets. Hand-fixing README on each release, as this PR does, stays the correct approach for that file; the site-side snippets don't have this constraint once #81 lands.

Test plan

• Doc-only change (README.md) — proofread the diff for accuracy per the repo's quality-gate policy; no Gradle build required.
• Confirmed 0.1.0-alpha.4 is the actual current released version via gradle.properties, .release-please-manifest.json, and the latest CHANGELOG.md entry.
• grep -n "0.1.0-alpha" README.md confirms no stale 0.1.0-alpha.1 references remain.

Part of #99 — the site-side portion needs #81 to merge first; filing/tracking that as follow-up rather than closing #99 outright here.

[OmarAlJarrah]

Superseded by #157, which switches the project to a stable 0.1.0 release line. The five README version occurrences this PR updated are incorporated there, retargeted from 0.1.0-alpha.4 to 0.1.0 (since the repo is graduating out of the alpha series entirely). Closing in favor of #157; #99 is tracked there.