Describe scheme variants in the versioning docs (#10747)

## Summary

The compression section of the versioning design still described
per-edition scheme configuration as planned work, and said that the
decimal scheme only declares the v1 wire ID. Since #10076, the decimal,
bit-packing and frame-of-reference schemes pick their v1 or v2 variant
from the allowed wire IDs. The section and the compression footnote of
the compatibility page now describe that behavior, with the decimal
scheme as the example. A v2 variant also writes the v1 ID, so a writer
must allow both IDs to use it.

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---------

Signed-off-by: Connor Tsui <connor.tsui20@gmail.com>

Author: connortsui20

docs/specs/versioning/compatibility.md

@@ -76,8 +76,7 @@ For the compatibility guarantee and minimum reader versions, see [Versioning](..
     support for Format B, so Library 1 still writes only Format A.
 
 [^compression]:
-    The default compressor filters schemes by their declared output wire IDs, but general per-writer
-    scheme configuration is not implemented. The planned configuration would select compatible
-    behavior before estimation, sampling, and full compression, avoiding recompression solely to
-    meet the edition. These cells still require the writer to construct a permitted representation.
-    See [Compression](design.md#compression) for the current behavior and planned work.
+    The default compressor picks its schemes and their variants from the permitted wire IDs before
+    it compresses, so it does not need to recompress only to meet the edition. These cells still
+    require the writer to construct a permitted representation. See
+    [Compression](design.md#compression) for the current behavior.

docs/specs/versioning/design.md

@@ -258,20 +258,21 @@ recording the minimum version.
 
 ## Compression
 
-The default BtrBlocks compressor excludes a scheme if any of its declared output wire IDs is
-forbidden. This filtering also applies to schemes used for child compression.
-
-The current decimal scheme produces only single-child arrays and declares the v1 wire ID, so values
-too wide for it remain in the standard uncompressed decimal representation. The multi-child
-serializer exists, but the default scheme does not construct those arrays and no declared edition
-permits their v2 ID.
-
-### Planned scheme configuration
-
-The planned improvement is to configure a scheme's behavior for the selected editions, allowing it
-to retain an older mode when its newer mode requires a forbidden format. That configuration needs to
-apply consistently to estimation, sampling, full compression, children, and fallbacks. General
-per-writer scheme configuration is not implemented, and its API is unsettled.
+The default BtrBlocks compressor uses only the schemes whose output it can write. A wire ID is
+allowed when the session registers it, the enabled editions permit it, and the compressor's preset
+does not exclude it. For example, the CUDA preset excludes some v2 IDs. The compressor excludes a
+scheme if any wire ID that the scheme declares is not allowed. Child arrays go through the same
+compressor. The writer still checks every serialized ID, so a forbidden ID in any child fails the
+write.
+
+A scheme can have variants that write different wire IDs. The compressor picks the variant from the
+allowed wire IDs, so callers do not choose it directly. For example, the decimal scheme uses its v2
+variant, which splits values too wide for one 64-bit child into several children, only when
+`vortex.decimal_byte_parts.v2` is allowed. Otherwise its v1 variant constructs only single-child
+arrays, and values too wide for one child remain in the standard uncompressed decimal
+representation. The v2 variant also writes the v1 ID for arrays that fit the v1 format, so allowing
+only the v2 ID excludes the scheme. No declared edition permits `vortex.decimal_byte_parts.v2` yet,
+so a writer that enforces editions writes only single-child decimal arrays.
 
 [^decimal-availability]:
     No declared edition currently permits the multi-child format. The