docs(compatibility): bump reference to v0.74.0, document Union/onpair/Variant gaps

The Rust reference implementation moved from v0.72.0 to v0.74.0 (two minor
releases) while this doc continued to claim v0.72.0 as the tested ceiling.
Audit findings against current Java code:

- `DType::Union` (`fbs.DType.Type.Union = 12`, added in Rust 0.71.0) is not
  decoded — `PostscriptParser.convertDType` switch has no Union case and falls
  through to the default branch, throwing `VortexException("unsupported DType
  typeType=12")`. The sealed `DType` interface in core has no `Union` variant
  either, so adding support is a multi-touch change.
- `vortex.onpair` (experimental in Rust 0.74.0, encodings/onpair/) is not
  registered. Files using it require `Registry.allowUnknown()` to open.
- `vortex.variant`: decode is complete (incl. shredded child); encode still
  throws `"encode not yet implemented"`. Rust 0.73.0 (#7945) added their write
  path + parquet-variant IO tests, widening the asymmetry.

Adds a new "Known wire-format gaps" section above the encoding table for
visibility; adds a row for `vortex.onpair`; rewords the Variant row to call
out the 0.73+ write-side asymmetry; flags the S3 fixture matrix as locked to
v0.72.0 and in need of a re-run when the v0.74.0 fixture set publishes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: dfa1

docs/compatibility.md

@@ -1,8 +1,17 @@
 # Compatibility
 
-Tested against the [Rust reference implementation](https://github.com/vortex-data/vortex) v0.72.0.
+Tested against the [Rust reference implementation](https://github.com/vortex-data/vortex) v0.74.0.
 For the rest of the API surface (reader, writer, scan, CLI), see [reference.md](reference.md).
 
+## Known wire-format gaps
+
+| Item | Introduced | Java status |
+|------|------------|-------------|
+| `DType::Union` (`fbs.DType.Type.Union = 12`) | Rust 0.71.0 | ❌ Decode throws `VortexException("unsupported DType typeType=12")`. No `DType.Union` variant in Java's sealed type. |
+| `vortex.onpair` experimental string encoding | Rust 0.74.0 | ❌ Not registered. Files using it fail to decode unless `Registry.allowUnknown()` is enabled. |
+| `vortex.variant` write path | Rust 0.73.0 (`Allow writing Variant to files`, #7945) | ❌ Java decode works; Java encode throws `"encode not yet implemented"`. Java→Rust round-trip not possible for Variant columns. |
+| Arrow extension array import affecting Variant shape | Rust 0.74.0 (#8125) | Untested. Re-run integration fixtures against v0.74.0 once published. |
+
 ## Encodings
 
 | Encoding ID                 | Class                      | Decode | Encode | Notes                                                                 |
@@ -39,7 +48,8 @@ For the rest of the API surface (reader, writer, scan, CLI), see [reference.md](
 | `fastlanes.for`             | `FrameOfReferenceEncoding` | ✅      | ✅      | Integer PTypes                                                        |
 | `fastlanes.rle`             | `RleEncoding`              | ✅      | ✅      | Chunk-based RLE                                                       |
 | `vortex.patched`            | `PatchedEncoding`          | ✅      | ❌      | Primitive PTypes; encode not yet implemented                          |
-| `vortex.variant`            | `VariantEncoding`          | ✅      | ❌      | Encode not yet implemented                                            |
+| `vortex.variant`            | `VariantEncoding`          | ✅      | ❌      | Decode (incl. shredded child); encode not yet implemented (Rust 0.73+) |
+| `vortex.onpair`             | _none_                     | ❌      | ❌      | Experimental in Rust 0.74.0; not yet ported                            |
 
 ### Unknown encodings
 
@@ -123,6 +133,10 @@ themselves.
 
 ## S3 Fixture Status (v0.72.0)
 
+> **Note:** the fixture matrix below is locked to `v0.72.0/`. The Rust reference is
+> now at `v0.74.0`; re-run the integration suite against `v0.74.0/arrays/` once
+> upstream publishes the corresponding fixture set, and refresh this section.
+
 Cross-language round-trips tested against Rust-written fixture files hosted at
 `s3://vortex-compat-fixtures/v0.72.0/arrays/`.