docs: add versioning policy

docs/source/about/versioning_policy.md

@@ -0,0 +1,118 @@
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+KIND, either express or implied.  See the License for the
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+
+# Versioning Policy
+
+Apache DataFusion Comet follows [semantic versioning](https://semver.org/) with the format
+`MAJOR.MINOR.PATCH`. This document describes what each component of a release means, what
+compatibility guarantees Comet provides, and how Comet relates to Apache Spark versions.
+
+This policy is inspired by, but is not identical to, the
+[Apache Spark versioning policy](https://spark.apache.org/versioning-policy.html). The differences
+reflect the fact that Comet is currently pre-1.0 and ships much more frequently than Spark.
+
+## Pre-1.0 Releases
+
+Comet is currently in the `0.x` series. Per semantic versioning, the `0.x` series is considered
+unstable:
+
+- A minor release (`0.X.0`) may include breaking changes.
+- A patch release (`0.X.Y`, where `Y > 0`) contains bug fixes only.
+
+In particular, the following are explicitly **not** stable in the `0.x` series and may change in any
+minor release without prior notice:
+
+- Configuration keys under `spark.comet.*` (names, defaults, and semantics).
+- The protobuf format used to serialize query plans between the JVM and the native library.
+- Internal Scala, Java, and Rust APIs that are not part of the documented public API.
+
+Where a breaking change is significant, it will be called out in the release notes.
+
+## Compatibility Commitments
+
+The following commitments apply to every Comet release, including the `0.x` series.
+
+### Public Scala and Java API
+
+Public classes and methods (for example, `org.apache.spark.CometPlugin`) are considered part of
+Comet's public API. Removing or making source- or binary-incompatible changes to a public API
+requires a deprecation cycle: the API must remain available, with a deprecation warning, for at
+least one minor release before removal.
+
+Public APIs annotated with `@Unstable` are exempt from this guarantee and may change in any minor
+release without a deprecation cycle. The `@Unstable` annotation does not yet exist and will be
+introduced as the need arises.
+
+### Query Result Semantics
+
+Expressions and operators whose support level is `Compatible` are expected to produce results that
+match Apache Spark. Result differences in `Compatible` items are tracked as bugs and fixed in
+subsequent releases.
+
+Items whose support level is `Incompatible` or `Unsupported` have no result-compatibility
+guarantees. `Incompatible` items require an explicit per-expression or per-operator opt-in
+(for example, `spark.comet.expr.<Name>.allowIncompatible=true`).
+
+For details on per-expression and per-operator support levels, see the
+[compatibility guide](../user-guide/latest/compatibility/index.md).
+
+## Apache Spark Version Support
+
+Each Comet release supports the **latest patch version** of every Apache Spark minor release that
+Comet targets at the time of release. The currently supported Spark versions are listed on the
+[Spark Version Compatibility](../user-guide/latest/compatibility/spark-versions.md) page.
+
+When the upstream Spark project publishes a new patch within a supported minor (for example,
+`3.5.8` → `3.5.9`), the next Comet release will pick it up. Older Spark patches within the same
+minor are not separately supported.
+
+Dropping support for an entire Spark minor (for example, removing Spark 3.4) is announced in the
+release notes of the **preceding** Comet minor release, giving users at least one release of
+notice before the change takes effect.
+
+Comet binaries are published per `(Spark minor × Scala binary version)` combination. Users must
+select the binary that matches their Spark and Scala installation.
+
+## Release Cadence
+
+Comet targets a `0.X.0` minor release every four to six weeks. Patch releases (`0.X.Y`) are made
+on demand, only when a critical bug or security fix needs to ship before the next minor release.
+
+Only the most recent minor release receives patch releases. Comet does not currently backport
+fixes to older minor releases; users are expected to upgrade forward.
+
+## Native Library Coupling
+
+Each Comet release ships a JVM jar and a native library that are built and tested together. The
+two artifacts must come from the **same Comet release**. Mixing a JVM jar from one Comet release
+with a native library from another is unsupported and may fail at runtime due to protobuf or FFI
+incompatibilities.
+
+## Road to 1.0
+
+When `1.0.0` ships:
+
+- Strict semantic versioning will apply to the public Scala and Java API: breaking changes will
+  only be made in a future major release. APIs annotated with `@Unstable` remain exempt and may
+  change in any minor release.
+- The stability commitments for configuration keys and the protobuf plan format will be
+  re-evaluated and documented as part of the `1.0.0` release.
+
+Tracking and planning for the `1.0.0` release happens in
+[issue #4082](https://github.com/apache/datafusion-comet/issues/4082).

docs/source/index.md

@@ -109,5 +109,6 @@ User Guide <user-guide/index>
 Contributor Guide <contributor-guide/index>
 Changelog <changelog/index>
 Comparison with Gluten <about/gluten_comparison>
+Versioning Policy <about/versioning_policy>
 ASF Links <asf/index>
 ```