v0.2.0-alpha.3

Supersedes the never-published v0.2.0-alpha.2 tag (Maven Central
rejected the re-publish after the original tag's deploy was cancelled
mid-flight). Content is identical to what alpha.2 would have shipped,
plus contributor credit in the [0.2.0-alpha.1] notes below.

Changed

• Integration tests now INSTALL quack; from the core repository,
  matching DuckDB v1.5.3 ("Variegata") where Quack ships as a signed
  core extension. The -unsigned flag and core_nightly repository are
  no longer required. The full 83-test suite passes against the
  quack extension bundled with DuckDB CLI v1.5.3 — no driver code
  changes needed, the wire format is unchanged.
• README and CLAUDE.md updated to reflect the v1.5.3 install path; the
  pre-1.5.3 INSTALL quack FROM core_nightly recipe is kept as a note
  for users on older builds.