feat: update lance dependency to v12.0.0 (#4216)

Targets `release/v0.39`, not `main`. That branch was cut from
`56bbd19f`, the last commit on `main` still pinned to lance 12, and
carries eight backported fixes; it will publish the next stable lancedb
on the lance 12.0.0 release.

## What changed

Rust workspace lance crates and `java/pom.xml` `lance-core` move from
`v12.0.0-beta.18` to
[`v12.0.0`](https://github.com/lance-format/lance/releases/tag/v12.0.0).
No compatibility fixes were needed.

Because the target is a stable lance, `ci/set_lance_version.py` takes
the `set_stable_version` path, so the workspace dependencies drop their
`git` and `tag` fields and resolve from crates.io. That switch is
required for the release itself — `cargo publish` rejects a crate with
git dependencies — and `Cargo.lock` now has zero `lance-format/lance`
references.

## Validation

* `python3 ci/validate_stable_lance.py` passes — the gate the `Create
Release Commit` workflow runs for a stable release.
* `cargo fmt --all`, `git diff --check` clean.
* All lance 12.0.0 crates confirmed published on crates.io, and
`org.lance:lance-core:12.0.0` on Maven Central.
* Full build and test verification is delegated to this PR's CI, which
also exercises the eight backported fixes on the branch.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

java/pom.xml

@@ -28,7 +28,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <arrow.version>15.0.0</arrow.version>
-        <lance-core.version>12.0.0-beta.18</lance-core.version>
+        <lance-core.version>12.0.0</lance-core.version>
         <spotless.skip>false</spotless.skip>
         <spotless.version>2.30.0</spotless.version>
         <spotless.java.googlejavaformat.version>1.7</spotless.java.googlejavaformat.version>